service backend

api/billing/fees/.air.toml

@@ -0,0 +1,32 @@
+# Config file for Air in TOML format
+
+root = "./../.."
+tmp_dir = "tmp"
+
+[build]
+cmd = "go build -o app -ldflags \"-X 'github.com/tech/sendico/billing/fees/internal/appversion.BuildUser=$(whoami)' -X 'github.com/tech/sendico/billing/fees/internal/appversion.Version=$APP_V' -X 'github.com/tech/sendico/billing/fees/internal/appversion.Branch=$BUILD_BRANCH' -X 'github.com/tech/sendico/billing/fees/internal/appversion.Revision=$GIT_REV' -X 'github.com/tech/sendico/billing/fees/internal/appversion.BuildDate=$(date)'\""
+bin = "./app"
+full_bin = "./app --debug --config.file=config.yml"
+include_ext = ["go", "yaml", "yml"]
+exclude_dir = ["billing/fees/tmp", "pkg/.git", "billing/fees/env"]
+exclude_regex = ["_test\\.go"]
+exclude_unchanged = true
+follow_symlink = true
+log = "air.log"
+delay = 0
+stop_on_error = true
+send_interrupt = true
+kill_delay = 500
+args_bin = []
+
+[log]
+time = false
+
+[color]
+main = "magenta"
+watcher = "cyan"
+build = "yellow"
+runner = "green"
+
+[misc]
+clean_on_exit = true

api/billing/fees/.gitignore

@@ -0,0 +1,3 @@
+internal/generated
+.gocache
+app

api/billing/fees/config.yml

@@ -0,0 +1,40 @@
+runtime:
+  shutdown_timeout_seconds: 15
+
+grpc:
+  network: tcp
+  address: ":50060"
+  enable_reflection: true
+  enable_health: true
+
+metrics:
+  address: ":9402"
+
+database:
+  driver: mongodb
+  settings:
+    host_env: FEES_MONGO_HOST
+    port_env: FEES_MONGO_PORT
+    database_env: FEES_MONGO_DATABASE
+    user_env: FEES_MONGO_USER
+    password_env: FEES_MONGO_PASSWORD
+    auth_source_env: FEES_MONGO_AUTH_SOURCE
+    replica_set_env: FEES_MONGO_REPLICA_SET
+
+messaging:
+  driver: NATS
+  settings:
+    url_env: NATS_URL
+    host_env: NATS_HOST
+    port_env: NATS_PORT
+    username_env: NATS_USER
+    password_env: NATS_PASSWORD
+    broker_name: Billing Fees Service
+    max_reconnects: 10
+    reconnect_wait: 5
+
+oracle:
+  address: "sendico_fx_oracle:50051"
+  dial_timeout_seconds: 5
+  call_timeout_seconds: 3
+  insecure: true

api/billing/fees/env/.gitignore

@@ -0,0 +1 @@
+.env.api

api/billing/fees/go.mod

@@ -0,0 +1,54 @@
+module github.com/tech/sendico/billing/fees
+
+go 1.25.3
+
+replace github.com/tech/sendico/pkg => ../../pkg
+
+replace github.com/tech/sendico/fx/oracle => ../../fx/oracle
+
+require (
+	github.com/tech/sendico/fx/oracle v0.0.0
+	github.com/tech/sendico/pkg v0.1.0
+	go.mongodb.org/mongo-driver v1.17.6
+	go.uber.org/zap v1.27.0
+	google.golang.org/grpc v1.76.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/bmatcuk/doublestar/v4 v4.9.1 // indirect
+	github.com/casbin/casbin/v2 v2.132.0 // indirect
+	github.com/casbin/govaluate v1.10.0 // indirect
+	github.com/casbin/mongodb-adapter/v3 v3.7.0 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/go-chi/chi/v5 v5.2.3 // indirect
+	github.com/golang/snappy v1.0.0 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/klauspost/compress v1.18.1 // indirect
+	github.com/mattn/go-colorable v0.1.14 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/montanaflynn/stats v0.7.1 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/nats-io/nats.go v1.47.0 // indirect
+	github.com/nats-io/nkeys v0.4.11 // indirect
+	github.com/nats-io/nuid v1.0.1 // indirect
+	github.com/prometheus/client_golang v1.23.2
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.67.2 // indirect
+	github.com/prometheus/procfs v0.19.2 // indirect
+	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
+	github.com/xdg-go/scram v1.1.2 // indirect
+	github.com/xdg-go/stringprep v1.0.4 // indirect
+	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
+	go.yaml.in/yaml/v2 v2.4.3 // indirect
+	golang.org/x/crypto v0.43.0 // indirect
+	golang.org/x/net v0.46.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+	golang.org/x/sys v0.37.0 // indirect
+	golang.org/x/text v0.30.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251103181224-f26f9409b101 // indirect
+	google.golang.org/protobuf v1.36.10
+)

api/billing/fees/go.sum

@@ -0,0 +1,225 @@
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api/billing/fees/internal/appversion/version.go

@@ -0,0 +1,28 @@
+package appversion
+
+import (
+	"github.com/tech/sendico/pkg/version"
+	vf "github.com/tech/sendico/pkg/version/factory"
+)
+
+// Build information populated at build time.
+var (
+	Version   string
+	Revision  string
+	Branch    string
+	BuildUser string
+	BuildDate string
+)
+
+// Create initialises a version.Printer with the build details for this service.
+func Create() version.Printer {
+	info := version.Info{
+		Program:   "Sendico Billing Fees Service",
+		Revision:  Revision,
+		Branch:    Branch,
+		BuildUser: BuildUser,
+		BuildDate: BuildDate,
+		Version:   Version,
+	}
+	return vf.Create(&info)
+}

api/billing/fees/internal/server/internal/serverimp.go

@@ -0,0 +1,163 @@
+package serverimp
+
+import (
+	"context"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/internal/service/fees"
+	"github.com/tech/sendico/billing/fees/storage"
+	mongostorage "github.com/tech/sendico/billing/fees/storage/mongo"
+	oracleclient "github.com/tech/sendico/fx/oracle/client"
+	"github.com/tech/sendico/pkg/api/routers"
+	"github.com/tech/sendico/pkg/db"
+	msg "github.com/tech/sendico/pkg/messaging"
+	"github.com/tech/sendico/pkg/mlogger"
+	"github.com/tech/sendico/pkg/server/grpcapp"
+	"go.uber.org/zap"
+	"gopkg.in/yaml.v3"
+)
+
+type Imp struct {
+	logger       mlogger.Logger
+	file         string
+	debug        bool
+	config       *config
+	app          *grpcapp.App[storage.Repository]
+	oracleClient oracleclient.Client
+}
+
+type config struct {
+	*grpcapp.Config `yaml:",inline"`
+	Oracle          OracleConfig `yaml:"oracle"`
+}
+
+type OracleConfig struct {
+	Address           string `yaml:"address"`
+	DialTimeoutSecs   int    `yaml:"dial_timeout_seconds"`
+	CallTimeoutSecs   int    `yaml:"call_timeout_seconds"`
+	InsecureTransport bool   `yaml:"insecure"`
+}
+
+func (c OracleConfig) dialTimeout() time.Duration {
+	if c.DialTimeoutSecs <= 0 {
+		return 5 * time.Second
+	}
+	return time.Duration(c.DialTimeoutSecs) * time.Second
+}
+
+func (c OracleConfig) callTimeout() time.Duration {
+	if c.CallTimeoutSecs <= 0 {
+		return 3 * time.Second
+	}
+	return time.Duration(c.CallTimeoutSecs) * time.Second
+}
+
+// Create initialises the billing fees server implementation.
+func Create(logger mlogger.Logger, file string, debug bool) (*Imp, error) {
+	return &Imp{
+		logger: logger.Named("server"),
+		file:   file,
+		debug:  debug,
+	}, nil
+}
+
+func (i *Imp) Shutdown() {
+	if i.app == nil {
+		if i.oracleClient != nil {
+			_ = i.oracleClient.Close()
+		}
+		return
+	}
+
+	timeout := 15 * time.Second
+	if i.config != nil && i.config.Runtime != nil {
+		timeout = i.config.Runtime.ShutdownTimeout()
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	i.app.Shutdown(ctx)
+	cancel()
+
+	if i.oracleClient != nil {
+		_ = i.oracleClient.Close()
+	}
+}
+
+func (i *Imp) Start() error {
+	cfg, err := i.loadConfig()
+	if err != nil {
+		return err
+	}
+	i.config = cfg
+
+	repoFactory := func(logger mlogger.Logger, conn *db.MongoConnection) (storage.Repository, error) {
+		return mongostorage.New(logger, conn)
+	}
+
+	var oracleClient oracleclient.Client
+	if addr := strings.TrimSpace(cfg.Oracle.Address); addr != "" {
+		dialCtx, cancel := context.WithTimeout(context.Background(), cfg.Oracle.dialTimeout())
+		defer cancel()
+
+		oc, err := oracleclient.New(dialCtx, oracleclient.Config{
+			Address:     addr,
+			DialTimeout: cfg.Oracle.dialTimeout(),
+			CallTimeout: cfg.Oracle.callTimeout(),
+			Insecure:    cfg.Oracle.InsecureTransport,
+		})
+		if err != nil {
+			i.logger.Warn("failed to initialise oracle client", zap.String("address", addr), zap.Error(err))
+		} else {
+			oracleClient = oc
+			i.oracleClient = oc
+			i.logger.Info("connected to oracle service", zap.String("address", addr))
+		}
+	}
+
+	serviceFactory := func(logger mlogger.Logger, repo storage.Repository, producer msg.Producer) (grpcapp.Service, error) {
+		opts := []fees.Option{}
+		if oracleClient != nil {
+			opts = append(opts, fees.WithOracleClient(oracleClient))
+		}
+		return fees.NewService(logger, repo, producer, opts...), nil
+	}
+
+	app, err := grpcapp.NewApp(i.logger, "billing_fees", cfg.Config, i.debug, repoFactory, serviceFactory)
+	if err != nil {
+		return err
+	}
+	i.app = app
+
+	return i.app.Start()
+}
+
+func (i *Imp) loadConfig() (*config, error) {
+	data, err := os.ReadFile(i.file)
+	if err != nil {
+		i.logger.Error("Could not read configuration file", zap.String("config_file", i.file), zap.Error(err))
+		return nil, err
+	}
+
+	cfg := &config{Config: &grpcapp.Config{}}
+	if err := yaml.Unmarshal(data, cfg); err != nil {
+		i.logger.Error("Failed to parse configuration", zap.Error(err))
+		return nil, err
+	}
+
+	if cfg.Runtime == nil {
+		cfg.Runtime = &grpcapp.RuntimeConfig{ShutdownTimeoutSeconds: 15}
+	}
+
+	if cfg.GRPC == nil {
+		cfg.GRPC = &routers.GRPCConfig{
+			Network:          "tcp",
+			Address:          ":50060",
+			EnableReflection: true,
+			EnableHealth:     true,
+		}
+	}
+
+	return cfg, nil
+}

api/billing/fees/internal/server/server.go

@@ -0,0 +1,12 @@
+package server
+
+import (
+	serverimp "github.com/tech/sendico/billing/fees/internal/server/internal"
+	"github.com/tech/sendico/pkg/mlogger"
+	"github.com/tech/sendico/pkg/server"
+)
+
+// Create constructs the billing fees server implementation.
+func Create(logger mlogger.Logger, file string, debug bool) (server.Application, error) {
+	return serverimp.Create(logger, file, debug)
+}

api/billing/fees/internal/service/fees/calculator.go

@@ -0,0 +1,449 @@
+package fees
+
+import (
+	"context"
+	"errors"
+	"math/big"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage/model"
+	oracleclient "github.com/tech/sendico/fx/oracle/client"
+	dmath "github.com/tech/sendico/pkg/decimal"
+	"github.com/tech/sendico/pkg/merrors"
+	"github.com/tech/sendico/pkg/mlogger"
+	feesv1 "github.com/tech/sendico/pkg/proto/billing/fees/v1"
+	accountingv1 "github.com/tech/sendico/pkg/proto/common/accounting/v1"
+	fxv1 "github.com/tech/sendico/pkg/proto/common/fx/v1"
+	moneyv1 "github.com/tech/sendico/pkg/proto/common/money/v1"
+	tracev1 "github.com/tech/sendico/pkg/proto/common/trace/v1"
+	"go.uber.org/zap"
+)
+
+// Calculator isolates fee rule evaluation logic so it can be reused and tested.
+type Calculator interface {
+	Compute(ctx context.Context, plan *model.FeePlan, intent *feesv1.Intent, bookedAt time.Time, trace *tracev1.TraceContext) (*CalculationResult, error)
+}
+
+// CalculationResult contains derived fee lines and audit metadata.
+type CalculationResult struct {
+	Lines   []*feesv1.DerivedPostingLine
+	Applied []*feesv1.AppliedRule
+	FxUsed  *feesv1.FXUsed
+}
+
+// quoteCalculator is the default Calculator implementation.
+type fxOracle interface {
+	LatestRate(ctx context.Context, req oracleclient.LatestRateParams) (*oracleclient.RateSnapshot, error)
+}
+
+type quoteCalculator struct {
+	logger mlogger.Logger
+	oracle fxOracle
+}
+
+func newQuoteCalculator(logger mlogger.Logger, oracle fxOracle) Calculator {
+	return &quoteCalculator{
+		logger: logger.Named("calculator"),
+		oracle: oracle,
+	}
+}
+
+func (c *quoteCalculator) Compute(ctx context.Context, plan *model.FeePlan, intent *feesv1.Intent, bookedAt time.Time, _ *tracev1.TraceContext) (*CalculationResult, error) {
+	if plan == nil {
+		return nil, merrors.InvalidArgument("plan is required")
+	}
+	if intent == nil {
+		return nil, merrors.InvalidArgument("intent is required")
+	}
+
+	trigger := convertTrigger(intent.GetTrigger())
+	if trigger == model.TriggerUnspecified {
+		return nil, merrors.InvalidArgument("unsupported trigger")
+	}
+
+	baseAmount, err := dmath.RatFromString(intent.GetBaseAmount().GetAmount())
+	if err != nil {
+		return nil, merrors.InvalidArgument("invalid base amount")
+	}
+	if baseAmount.Sign() < 0 {
+		return nil, merrors.InvalidArgument("base amount cannot be negative")
+	}
+
+	baseScale := inferScale(intent.GetBaseAmount().GetAmount())
+
+	rules := make([]model.FeeRule, len(plan.Rules))
+	copy(rules, plan.Rules)
+	sort.SliceStable(rules, func(i, j int) bool {
+		if rules[i].Priority == rules[j].Priority {
+			return rules[i].RuleID < rules[j].RuleID
+		}
+		return rules[i].Priority < rules[j].Priority
+	})
+
+	lines := make([]*feesv1.DerivedPostingLine, 0, len(rules))
+	applied := make([]*feesv1.AppliedRule, 0, len(rules))
+
+	planID := ""
+	if planRef := plan.GetID(); planRef != nil && !planRef.IsZero() {
+		planID = planRef.Hex()
+	}
+
+	for _, rule := range rules {
+		if !shouldApplyRule(rule, trigger, intent.GetAttributes(), bookedAt) {
+			continue
+		}
+
+		ledgerAccountRef := strings.TrimSpace(rule.LedgerAccountRef)
+		if ledgerAccountRef == "" {
+			c.logger.Warn("fee rule missing ledger account reference", zap.String("rule_id", rule.RuleID))
+			continue
+		}
+
+		amount, scale, calcErr := c.calculateRuleAmount(baseAmount, baseScale, rule)
+		if calcErr != nil {
+			if !errors.Is(calcErr, merrors.ErrInvalidArg) {
+				c.logger.Warn("failed to calculate fee rule amount", zap.String("rule_id", rule.RuleID), zap.Error(calcErr))
+			}
+			continue
+		}
+		if amount.Sign() == 0 {
+			continue
+		}
+
+		currency := intent.GetBaseAmount().GetCurrency()
+		if override := strings.TrimSpace(rule.Currency); override != "" {
+			currency = override
+		}
+
+		entrySide := mapEntrySide(rule.EntrySide)
+		if entrySide == accountingv1.EntrySide_ENTRY_SIDE_UNSPECIFIED {
+			entrySide = accountingv1.EntrySide_ENTRY_SIDE_CREDIT
+		}
+
+		meta := map[string]string{
+			"fee_rule_id": rule.RuleID,
+		}
+		if planID != "" {
+			meta["fee_plan_id"] = planID
+		}
+		if rule.Metadata != nil {
+			if taxCode := strings.TrimSpace(rule.Metadata["tax_code"]); taxCode != "" {
+				meta["tax_code"] = taxCode
+			}
+			if taxRate := strings.TrimSpace(rule.Metadata["tax_rate"]); taxRate != "" {
+				meta["tax_rate"] = taxRate
+			}
+		}
+
+		lines = append(lines, &feesv1.DerivedPostingLine{
+			LedgerAccountRef: ledgerAccountRef,
+			Money: &moneyv1.Money{
+				Amount:   dmath.FormatRat(amount, scale),
+				Currency: currency,
+			},
+			LineType: mapLineType(rule.LineType),
+			Side:     entrySide,
+			Meta:     meta,
+		})
+
+		applied = append(applied, &feesv1.AppliedRule{
+			RuleId:      rule.RuleID,
+			RuleVersion: planID,
+			Formula:     rule.Formula,
+			Rounding:    mapRoundingMode(rule.Rounding),
+			TaxCode:     metadataValue(rule.Metadata, "tax_code"),
+			TaxRate:     metadataValue(rule.Metadata, "tax_rate"),
+			Parameters:  cloneStringMap(rule.Metadata),
+		})
+	}
+
+	var fxUsed *feesv1.FXUsed
+	if trigger == model.TriggerFXConversion && c.oracle != nil {
+		fxUsed = c.buildFxUsed(ctx, intent)
+	}
+
+	return &CalculationResult{
+		Lines:   lines,
+		Applied: applied,
+		FxUsed:  fxUsed,
+	}, nil
+}
+
+func (c *quoteCalculator) calculateRuleAmount(baseAmount *big.Rat, baseScale uint32, rule model.FeeRule) (*big.Rat, uint32, error) {
+	scale, err := resolveRuleScale(rule, baseScale)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	result := new(big.Rat)
+
+	if percentage := strings.TrimSpace(rule.Percentage); percentage != "" {
+		percentageRat, perr := dmath.RatFromString(percentage)
+		if perr != nil {
+			return nil, 0, merrors.InvalidArgument("invalid percentage")
+		}
+		result = dmath.AddRat(result, dmath.MulRat(baseAmount, percentageRat))
+	}
+
+	if fixed := strings.TrimSpace(rule.FixedAmount); fixed != "" {
+		fixedRat, ferr := dmath.RatFromString(fixed)
+		if ferr != nil {
+			return nil, 0, merrors.InvalidArgument("invalid fixed amount")
+		}
+		result = dmath.AddRat(result, fixedRat)
+	}
+
+	if minStr := strings.TrimSpace(rule.MinimumAmount); minStr != "" {
+		minRat, merr := dmath.RatFromString(minStr)
+		if merr != nil {
+			return nil, 0, merrors.InvalidArgument("invalid minimum amount")
+		}
+		if dmath.CmpRat(result, minRat) < 0 {
+			result = new(big.Rat).Set(minRat)
+		}
+	}
+
+	if maxStr := strings.TrimSpace(rule.MaximumAmount); maxStr != "" {
+		maxRat, merr := dmath.RatFromString(maxStr)
+		if merr != nil {
+			return nil, 0, merrors.InvalidArgument("invalid maximum amount")
+		}
+		if dmath.CmpRat(result, maxRat) > 0 {
+			result = new(big.Rat).Set(maxRat)
+		}
+	}
+
+	if result.Sign() < 0 {
+		result = new(big.Rat).Abs(result)
+	}
+
+	rounded, rerr := dmath.RoundRatToScale(result, scale, toDecimalRounding(rule.Rounding))
+	if rerr != nil {
+		return nil, 0, rerr
+	}
+
+	return rounded, scale, nil
+}
+
+const (
+	attrFxBaseCurrency  = "fx_base_currency"
+	attrFxQuoteCurrency = "fx_quote_currency"
+	attrFxProvider      = "fx_provider"
+	attrFxSide          = "fx_side"
+	attrFxRateOverride  = "fx_rate"
+)
+
+func (c *quoteCalculator) buildFxUsed(ctx context.Context, intent *feesv1.Intent) *feesv1.FXUsed {
+	if intent == nil || c.oracle == nil {
+		return nil
+	}
+
+	attrs := intent.GetAttributes()
+	base := strings.TrimSpace(attrs[attrFxBaseCurrency])
+	quote := strings.TrimSpace(attrs[attrFxQuoteCurrency])
+	if base == "" || quote == "" {
+		return nil
+	}
+
+	pair := &fxv1.CurrencyPair{Base: base, Quote: quote}
+	provider := strings.TrimSpace(attrs[attrFxProvider])
+
+	snapshot, err := c.oracle.LatestRate(ctx, oracleclient.LatestRateParams{
+		Meta:     oracleclient.RequestMeta{},
+		Pair:     pair,
+		Provider: provider,
+	})
+	if err != nil {
+		c.logger.Warn("fees: failed to fetch FX context", zap.Error(err))
+		return nil
+	}
+	if snapshot == nil {
+		return nil
+	}
+
+	rateValue := strings.TrimSpace(attrs[attrFxRateOverride])
+	if rateValue == "" {
+		rateValue = snapshot.Mid
+	}
+	if rateValue == "" {
+		rateValue = snapshot.Ask
+	}
+	if rateValue == "" {
+		rateValue = snapshot.Bid
+	}
+
+	return &feesv1.FXUsed{
+		Pair:       pair,
+		Side:       parseFxSide(strings.TrimSpace(attrs[attrFxSide])),
+		Rate:       &moneyv1.Decimal{Value: rateValue},
+		AsofUnixMs: snapshot.AsOf.UnixMilli(),
+		Provider:   snapshot.Provider,
+		RateRef:    snapshot.RateRef,
+		SpreadBps:  &moneyv1.Decimal{Value: snapshot.SpreadBps},
+	}
+}
+
+func parseFxSide(value string) fxv1.Side {
+	switch strings.ToLower(value) {
+	case "buy_base", "buy_base_sell_quote", "buy":
+		return fxv1.Side_BUY_BASE_SELL_QUOTE
+	case "sell_base", "sell_base_buy_quote", "sell":
+		return fxv1.Side_SELL_BASE_BUY_QUOTE
+	default:
+		return fxv1.Side_SIDE_UNSPECIFIED
+	}
+}
+
+func inferScale(amount string) uint32 {
+	value := strings.TrimSpace(amount)
+	if value == "" {
+		return 0
+	}
+	if idx := strings.IndexAny(value, "eE"); idx >= 0 {
+		value = value[:idx]
+	}
+	if strings.HasPrefix(value, "+") || strings.HasPrefix(value, "-") {
+		value = value[1:]
+	}
+	if dot := strings.IndexByte(value, '.'); dot >= 0 {
+		return uint32(len(value[dot+1:]))
+	}
+	return 0
+}
+
+func shouldApplyRule(rule model.FeeRule, trigger model.Trigger, attributes map[string]string, bookedAt time.Time) bool {
+	if rule.Trigger != trigger {
+		return false
+	}
+	if rule.EffectiveFrom.After(bookedAt) {
+		return false
+	}
+	if rule.EffectiveTo != nil && rule.EffectiveTo.Before(bookedAt) {
+		return false
+	}
+	return ruleMatchesAttributes(rule, attributes)
+}
+
+func resolveRuleScale(rule model.FeeRule, fallback uint32) (uint32, error) {
+	if rule.Metadata != nil {
+		for _, field := range []string{"scale", "decimals", "precision"} {
+			if value, ok := rule.Metadata[field]; ok && strings.TrimSpace(value) != "" {
+				return parseScale(field, value)
+			}
+		}
+	}
+	return fallback, nil
+}
+
+func parseScale(field, value string) (uint32, error) {
+	clean := strings.TrimSpace(value)
+	if clean == "" {
+		return 0, merrors.InvalidArgument(field + " is empty")
+	}
+	parsed, err := strconv.ParseUint(clean, 10, 32)
+	if err != nil {
+		return 0, merrors.InvalidArgument("invalid " + field + " value")
+	}
+	return uint32(parsed), nil
+}
+
+func metadataValue(meta map[string]string, key string) string {
+	if meta == nil {
+		return ""
+	}
+	return strings.TrimSpace(meta[key])
+}
+
+func cloneStringMap(src map[string]string) map[string]string {
+	if len(src) == 0 {
+		return nil
+	}
+	cloned := make(map[string]string, len(src))
+	for k, v := range src {
+		cloned[k] = v
+	}
+	return cloned
+}
+
+func ruleMatchesAttributes(rule model.FeeRule, attributes map[string]string) bool {
+	if len(rule.AppliesTo) == 0 {
+		return true
+	}
+	for key, value := range rule.AppliesTo {
+		if attributes == nil {
+			return false
+		}
+		if attrValue, ok := attributes[key]; !ok || attrValue != value {
+			return false
+		}
+	}
+	return true
+}
+
+func convertTrigger(trigger feesv1.Trigger) model.Trigger {
+	switch trigger {
+	case feesv1.Trigger_TRIGGER_CAPTURE:
+		return model.TriggerCapture
+	case feesv1.Trigger_TRIGGER_REFUND:
+		return model.TriggerRefund
+	case feesv1.Trigger_TRIGGER_DISPUTE:
+		return model.TriggerDispute
+	case feesv1.Trigger_TRIGGER_PAYOUT:
+		return model.TriggerPayout
+	case feesv1.Trigger_TRIGGER_FX_CONVERSION:
+		return model.TriggerFXConversion
+	default:
+		return model.TriggerUnspecified
+	}
+}
+
+func mapLineType(lineType string) accountingv1.PostingLineType {
+	switch strings.ToLower(lineType) {
+	case "tax":
+		return accountingv1.PostingLineType_POSTING_LINE_TAX
+	case "spread":
+		return accountingv1.PostingLineType_POSTING_LINE_SPREAD
+	case "reversal":
+		return accountingv1.PostingLineType_POSTING_LINE_REVERSAL
+	default:
+		return accountingv1.PostingLineType_POSTING_LINE_FEE
+	}
+}
+
+func mapEntrySide(entrySide string) accountingv1.EntrySide {
+	switch strings.ToLower(entrySide) {
+	case "debit":
+		return accountingv1.EntrySide_ENTRY_SIDE_DEBIT
+	case "credit":
+		return accountingv1.EntrySide_ENTRY_SIDE_CREDIT
+	default:
+		return accountingv1.EntrySide_ENTRY_SIDE_UNSPECIFIED
+	}
+}
+
+func toDecimalRounding(mode string) dmath.RoundingMode {
+	switch strings.ToLower(strings.TrimSpace(mode)) {
+	case "half_up":
+		return dmath.RoundingModeHalfUp
+	case "down":
+		return dmath.RoundingModeDown
+	case "half_even", "bankers":
+		return dmath.RoundingModeHalfEven
+	default:
+		return dmath.RoundingModeHalfEven
+	}
+}
+
+func mapRoundingMode(mode string) moneyv1.RoundingMode {
+	switch strings.ToLower(mode) {
+	case "half_up":
+		return moneyv1.RoundingMode_ROUND_HALF_UP
+	case "down":
+		return moneyv1.RoundingMode_ROUND_DOWN
+	default:
+		return moneyv1.RoundingMode_ROUND_HALF_EVEN
+	}
+}

api/billing/fees/internal/service/fees/metrics.go

@@ -0,0 +1,71 @@
+package fees
+
+import (
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	feesv1 "github.com/tech/sendico/pkg/proto/billing/fees/v1"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+var (
+	metricsOnce sync.Once
+
+	quoteRequestsTotal *prometheus.CounterVec
+	quoteLatency       *prometheus.HistogramVec
+)
+
+func initMetrics() {
+	metricsOnce.Do(func() {
+		quoteRequestsTotal = promauto.NewCounterVec(
+			prometheus.CounterOpts{
+				Namespace: "billing",
+				Subsystem: "fees",
+				Name:      "requests_total",
+				Help:      "Total number of fee service requests processed.",
+			},
+			[]string{"call", "trigger", "status", "fx_used"},
+		)
+
+		quoteLatency = promauto.NewHistogramVec(
+			prometheus.HistogramOpts{
+				Namespace: "billing",
+				Subsystem: "fees",
+				Name:      "request_latency_seconds",
+				Help:      "Latency of fee service requests.",
+				Buckets:   prometheus.DefBuckets,
+			},
+			[]string{"call", "trigger", "status", "fx_used"},
+		)
+	})
+}
+
+func observeMetrics(call string, trigger feesv1.Trigger, statusLabel string, fxUsed bool, took time.Duration) {
+	triggerLabel := trigger.String()
+	if trigger == feesv1.Trigger_TRIGGER_UNSPECIFIED {
+		triggerLabel = "TRIGGER_UNSPECIFIED"
+	}
+	fxLabel := strconv.FormatBool(fxUsed)
+	quoteRequestsTotal.WithLabelValues(call, triggerLabel, statusLabel, fxLabel).Inc()
+	quoteLatency.WithLabelValues(call, triggerLabel, statusLabel, fxLabel).Observe(took.Seconds())
+}
+
+func statusFromError(err error) string {
+	if err == nil {
+		return "success"
+	}
+	st, ok := status.FromError(err)
+	if !ok {
+		return "error"
+	}
+	code := st.Code()
+	if code == codes.OK {
+		return "success"
+	}
+	return strings.ToLower(code.String())
+}

api/billing/fees/internal/service/fees/options.go

@@ -0,0 +1,37 @@
+package fees
+
+import (
+	oracleclient "github.com/tech/sendico/fx/oracle/client"
+	clockpkg "github.com/tech/sendico/pkg/clock"
+)
+
+// Option configures a Service instance.
+type Option func(*Service)
+
+// WithClock sets a custom clock implementation.
+func WithClock(clock clockpkg.Clock) Option {
+	return func(s *Service) {
+		if clock != nil {
+			s.clock = clock
+		}
+	}
+}
+
+// WithCalculator sets a custom calculator implementation.
+func WithCalculator(calculator Calculator) Option {
+	return func(s *Service) {
+		if calculator != nil {
+			s.calculator = calculator
+		}
+	}
+}
+
+// WithOracleClient wires an FX oracle client for FX trigger evaluations.
+func WithOracleClient(oracle oracleclient.Client) Option {
+	return func(s *Service) {
+		s.oracle = oracle
+		if qc, ok := s.calculator.(*quoteCalculator); ok {
+			qc.oracle = oracle
+		}
+	}
+}

api/billing/fees/internal/service/fees/service.go

@@ -0,0 +1,322 @@
+package fees
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"strings"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage"
+	oracleclient "github.com/tech/sendico/fx/oracle/client"
+	"github.com/tech/sendico/pkg/api/routers"
+	clockpkg "github.com/tech/sendico/pkg/clock"
+	"github.com/tech/sendico/pkg/merrors"
+	msg "github.com/tech/sendico/pkg/messaging"
+	"github.com/tech/sendico/pkg/mlogger"
+	feesv1 "github.com/tech/sendico/pkg/proto/billing/fees/v1"
+	tracev1 "github.com/tech/sendico/pkg/proto/common/trace/v1"
+	"go.mongodb.org/mongo-driver/bson/primitive"
+	"go.uber.org/zap"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/types/known/timestamppb"
+)
+
+type Service struct {
+	logger     mlogger.Logger
+	storage    storage.Repository
+	producer   msg.Producer
+	clock      clockpkg.Clock
+	calculator Calculator
+	oracle     oracleclient.Client
+	feesv1.UnimplementedFeeEngineServer
+}
+
+func NewService(logger mlogger.Logger, repo storage.Repository, producer msg.Producer, opts ...Option) *Service {
+	svc := &Service{
+		logger:   logger.Named("fees"),
+		storage:  repo,
+		producer: producer,
+		clock:    clockpkg.NewSystem(),
+	}
+	initMetrics()
+
+	for _, opt := range opts {
+		opt(svc)
+	}
+
+	if svc.clock == nil {
+		svc.clock = clockpkg.NewSystem()
+	}
+	if svc.calculator == nil {
+		svc.calculator = newQuoteCalculator(svc.logger, svc.oracle)
+	}
+
+	return svc
+}
+
+func (s *Service) Register(router routers.GRPC) error {
+	return router.Register(func(reg grpc.ServiceRegistrar) {
+		feesv1.RegisterFeeEngineServer(reg, s)
+	})
+}
+
+func (s *Service) QuoteFees(ctx context.Context, req *feesv1.QuoteFeesRequest) (resp *feesv1.QuoteFeesResponse, err error) {
+	start := s.clock.Now()
+	trigger := feesv1.Trigger_TRIGGER_UNSPECIFIED
+	if req != nil && req.GetIntent() != nil {
+		trigger = req.GetIntent().GetTrigger()
+	}
+	var fxUsed bool
+	defer func() {
+		statusLabel := statusFromError(err)
+		if err == nil && resp != nil {
+			fxUsed = resp.GetFxUsed() != nil
+		}
+		observeMetrics("quote", trigger, statusLabel, fxUsed, time.Since(start))
+	}()
+
+	if err = s.validateQuoteRequest(req); err != nil {
+		return nil, err
+	}
+
+	orgRef, parseErr := primitive.ObjectIDFromHex(req.GetMeta().GetOrganizationRef())
+	if parseErr != nil {
+		err = status.Error(codes.InvalidArgument, "invalid organization_ref")
+		return nil, err
+	}
+
+	lines, applied, fx, computeErr := s.computeQuote(ctx, orgRef, req.GetIntent(), req.GetPolicy(), req.GetMeta().GetTrace())
+	if computeErr != nil {
+		err = computeErr
+		return nil, err
+	}
+
+	resp = &feesv1.QuoteFeesResponse{
+		Meta:    &feesv1.ResponseMeta{Trace: req.GetMeta().GetTrace()},
+		Lines:   lines,
+		Applied: applied,
+		FxUsed:  fx,
+	}
+	return resp, nil
+}
+
+func (s *Service) PrecomputeFees(ctx context.Context, req *feesv1.PrecomputeFeesRequest) (resp *feesv1.PrecomputeFeesResponse, err error) {
+	start := s.clock.Now()
+	trigger := feesv1.Trigger_TRIGGER_UNSPECIFIED
+	if req != nil && req.GetIntent() != nil {
+		trigger = req.GetIntent().GetTrigger()
+	}
+	var fxUsed bool
+	defer func() {
+		statusLabel := statusFromError(err)
+		if err == nil && resp != nil {
+			fxUsed = resp.GetFxUsed() != nil
+		}
+		observeMetrics("precompute", trigger, statusLabel, fxUsed, time.Since(start))
+	}()
+
+	if err = s.validatePrecomputeRequest(req); err != nil {
+		return nil, err
+	}
+
+	now := s.clock.Now()
+
+	orgRef, parseErr := primitive.ObjectIDFromHex(req.GetMeta().GetOrganizationRef())
+	if parseErr != nil {
+		err = status.Error(codes.InvalidArgument, "invalid organization_ref")
+		return nil, err
+	}
+
+	lines, applied, fx, computeErr := s.computeQuoteWithTime(ctx, orgRef, req.GetIntent(), nil, req.GetMeta().GetTrace(), now)
+	if computeErr != nil {
+		err = computeErr
+		return nil, err
+	}
+
+	ttl := req.GetTtlMs()
+	if ttl <= 0 {
+		ttl = 60000
+	}
+	expiresAt := now.Add(time.Duration(ttl) * time.Millisecond)
+
+	payload := feeQuoteTokenPayload{
+		OrganizationRef: req.GetMeta().GetOrganizationRef(),
+		Intent:          req.GetIntent(),
+		ExpiresAtUnixMs: expiresAt.UnixMilli(),
+		Trace:           req.GetMeta().GetTrace(),
+	}
+
+	var token string
+	if token, err = encodeTokenPayload(payload); err != nil {
+		s.logger.Warn("failed to encode fee quote token", zap.Error(err))
+		err = status.Error(codes.Internal, "failed to encode fee quote token")
+		return nil, err
+	}
+
+	resp = &feesv1.PrecomputeFeesResponse{
+		Meta:          &feesv1.ResponseMeta{Trace: req.GetMeta().GetTrace()},
+		FeeQuoteToken: token,
+		ExpiresAt:     timestamppb.New(expiresAt),
+		Lines:         lines,
+		Applied:       applied,
+		FxUsed:        fx,
+	}
+	return resp, nil
+}
+
+func (s *Service) ValidateFeeToken(ctx context.Context, req *feesv1.ValidateFeeTokenRequest) (resp *feesv1.ValidateFeeTokenResponse, err error) {
+	start := s.clock.Now()
+	trigger := feesv1.Trigger_TRIGGER_UNSPECIFIED
+	var fxUsed bool
+	defer func() {
+		statusLabel := statusFromError(err)
+		if err == nil && resp != nil {
+			if !resp.GetValid() {
+				statusLabel = "invalid"
+			}
+			fxUsed = resp.GetFxUsed() != nil
+			if resp.GetIntent() != nil {
+				trigger = resp.GetIntent().GetTrigger()
+			}
+		}
+		observeMetrics("validate", trigger, statusLabel, fxUsed, time.Since(start))
+	}()
+
+	if req == nil || strings.TrimSpace(req.GetFeeQuoteToken()) == "" {
+		err = status.Error(codes.InvalidArgument, "fee_quote_token is required")
+		return nil, err
+	}
+
+	now := s.clock.Now()
+
+	payload, decodeErr := decodeTokenPayload(req.GetFeeQuoteToken())
+	if decodeErr != nil {
+		s.logger.Warn("failed to decode fee quote token", zap.Error(decodeErr))
+		resp = &feesv1.ValidateFeeTokenResponse{Meta: &feesv1.ResponseMeta{}, Valid: false, Reason: "invalid_token"}
+		return resp, nil
+	}
+
+	trigger = payload.Intent.GetTrigger()
+
+	if now.UnixMilli() > payload.ExpiresAtUnixMs {
+		resp = &feesv1.ValidateFeeTokenResponse{Meta: &feesv1.ResponseMeta{}, Valid: false, Reason: "expired"}
+		return resp, nil
+	}
+
+	orgRef, parseErr := primitive.ObjectIDFromHex(payload.OrganizationRef)
+	if parseErr != nil {
+		s.logger.Warn("token contained invalid organization reference", zap.Error(parseErr))
+		resp = &feesv1.ValidateFeeTokenResponse{Meta: &feesv1.ResponseMeta{}, Valid: false, Reason: "invalid_token"}
+		return resp, nil
+	}
+
+	lines, applied, fx, computeErr := s.computeQuoteWithTime(ctx, orgRef, payload.Intent, nil, payload.Trace, now)
+	if computeErr != nil {
+		err = computeErr
+		return nil, err
+	}
+
+	resp = &feesv1.ValidateFeeTokenResponse{
+		Meta:    &feesv1.ResponseMeta{Trace: payload.Trace},
+		Valid:   true,
+		Intent:  payload.Intent,
+		Lines:   lines,
+		Applied: applied,
+		FxUsed:  fx,
+	}
+	return resp, nil
+}
+
+func (s *Service) validateQuoteRequest(req *feesv1.QuoteFeesRequest) error {
+	if req == nil {
+		return status.Error(codes.InvalidArgument, "request is required")
+	}
+	if req.GetMeta() == nil || strings.TrimSpace(req.GetMeta().GetOrganizationRef()) == "" {
+		return status.Error(codes.InvalidArgument, "meta.organization_ref is required")
+	}
+	if req.GetIntent() == nil {
+		return status.Error(codes.InvalidArgument, "intent is required")
+	}
+	if req.GetIntent().GetTrigger() == feesv1.Trigger_TRIGGER_UNSPECIFIED {
+		return status.Error(codes.InvalidArgument, "intent.trigger is required")
+	}
+	if req.GetIntent().GetBaseAmount() == nil {
+		return status.Error(codes.InvalidArgument, "intent.base_amount is required")
+	}
+	if strings.TrimSpace(req.GetIntent().GetBaseAmount().GetAmount()) == "" {
+		return status.Error(codes.InvalidArgument, "intent.base_amount.amount is required")
+	}
+	if strings.TrimSpace(req.GetIntent().GetBaseAmount().GetCurrency()) == "" {
+		return status.Error(codes.InvalidArgument, "intent.base_amount.currency is required")
+	}
+	return nil
+}
+
+func (s *Service) validatePrecomputeRequest(req *feesv1.PrecomputeFeesRequest) error {
+	if req == nil {
+		return status.Error(codes.InvalidArgument, "request is required")
+	}
+	return s.validateQuoteRequest(&feesv1.QuoteFeesRequest{Meta: req.GetMeta(), Intent: req.GetIntent()})
+}
+
+func (s *Service) computeQuote(ctx context.Context, orgRef primitive.ObjectID, intent *feesv1.Intent, overrides *feesv1.PolicyOverrides, trace *tracev1.TraceContext) ([]*feesv1.DerivedPostingLine, []*feesv1.AppliedRule, *feesv1.FXUsed, error) {
+	return s.computeQuoteWithTime(ctx, orgRef, intent, overrides, trace, s.clock.Now())
+}
+
+func (s *Service) computeQuoteWithTime(ctx context.Context, orgRef primitive.ObjectID, intent *feesv1.Intent, overrides *feesv1.PolicyOverrides, trace *tracev1.TraceContext, now time.Time) ([]*feesv1.DerivedPostingLine, []*feesv1.AppliedRule, *feesv1.FXUsed, error) {
+	bookedAt := now
+	if intent.GetBookedAt() != nil && intent.GetBookedAt().IsValid() {
+		bookedAt = intent.GetBookedAt().AsTime()
+	}
+
+	plan, err := s.storage.Plans().GetActivePlan(ctx, orgRef, bookedAt)
+	if err != nil {
+		if errors.Is(err, storage.ErrFeePlanNotFound) {
+			return nil, nil, nil, status.Error(codes.NotFound, "fee plan not found")
+		}
+		s.logger.Warn("failed to load active fee plan", zap.Error(err))
+		return nil, nil, nil, status.Error(codes.Internal, "failed to load fee plan")
+	}
+
+	result, calcErr := s.calculator.Compute(ctx, plan, intent, bookedAt, trace)
+	if calcErr != nil {
+		if errors.Is(calcErr, merrors.ErrInvalidArg) {
+			return nil, nil, nil, status.Error(codes.InvalidArgument, calcErr.Error())
+		}
+		s.logger.Warn("failed to compute fee quote", zap.Error(calcErr))
+		return nil, nil, nil, status.Error(codes.Internal, "failed to compute fee quote")
+	}
+
+	return result.Lines, result.Applied, result.FxUsed, nil
+}
+
+type feeQuoteTokenPayload struct {
+	OrganizationRef string                `json:"organization_ref"`
+	Intent          *feesv1.Intent        `json:"intent"`
+	ExpiresAtUnixMs int64                 `json:"expires_at_unix_ms"`
+	Trace           *tracev1.TraceContext `json:"trace,omitempty"`
+}
+
+func encodeTokenPayload(payload feeQuoteTokenPayload) (string, error) {
+	data, err := json.Marshal(payload)
+	if err != nil {
+		return "", merrors.Internal("fees: failed to serialize token payload")
+	}
+	return base64.StdEncoding.EncodeToString(data), nil
+}
+
+func decodeTokenPayload(token string) (feeQuoteTokenPayload, error) {
+	var payload feeQuoteTokenPayload
+	data, err := base64.StdEncoding.DecodeString(token)
+	if err != nil {
+		return payload, merrors.InvalidArgument("fees: invalid token encoding")
+	}
+	if err := json.Unmarshal(data, &payload); err != nil {
+		return payload, merrors.InvalidArgument("fees: invalid token payload")
+	}
+	return payload, nil
+}

api/billing/fees/internal/service/fees/service_test.go

@@ -0,0 +1,476 @@
+package fees
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage"
+	"github.com/tech/sendico/billing/fees/storage/model"
+	oracleclient "github.com/tech/sendico/fx/oracle/client"
+	me "github.com/tech/sendico/pkg/messaging/envelope"
+	feesv1 "github.com/tech/sendico/pkg/proto/billing/fees/v1"
+	moneyv1 "github.com/tech/sendico/pkg/proto/common/money/v1"
+	tracev1 "github.com/tech/sendico/pkg/proto/common/trace/v1"
+	"go.mongodb.org/mongo-driver/bson/primitive"
+	"go.uber.org/zap"
+	"google.golang.org/protobuf/types/known/timestamppb"
+)
+
+func TestQuoteFees_ComputesDerivedLines(t *testing.T) {
+	t.Helper()
+
+	now := time.Date(2024, 1, 10, 16, 0, 0, 0, time.UTC)
+	orgRef := primitive.NewObjectID()
+
+	plan := &model.FeePlan{
+		Active:        true,
+		EffectiveFrom: now.Add(-time.Hour),
+		Rules: []model.FeeRule{
+			{
+				RuleID:           "capture_default",
+				Trigger:          model.TriggerCapture,
+				Priority:         10,
+				Percentage:       "0.029",
+				FixedAmount:      "0.30",
+				LedgerAccountRef: "acct:fees",
+				LineType:         "fee",
+				EntrySide:        "credit",
+				Rounding:         "half_up",
+				Metadata: map[string]string{
+					"scale":    "2",
+					"tax_code": "VAT",
+					"tax_rate": "0.20",
+				},
+				EffectiveFrom: now.Add(-time.Hour),
+			},
+		},
+	}
+	plan.SetID(primitive.NewObjectID())
+	plan.SetOrganizationRef(orgRef)
+
+	service := NewService(
+		zap.NewNop(),
+		&stubRepository{plans: &stubPlansStore{plan: plan}},
+		noopProducer{},
+		WithClock(fixedClock{now: now}),
+	)
+
+	req := &feesv1.QuoteFeesRequest{
+		Meta: &feesv1.RequestMeta{
+			OrganizationRef: orgRef.Hex(),
+			Trace: &tracev1.TraceContext{
+				TraceRef: "trace-capture",
+			},
+		},
+		Intent: &feesv1.Intent{
+			Trigger: feesv1.Trigger_TRIGGER_CAPTURE,
+			BaseAmount: &moneyv1.Money{
+				Amount:   "100.00",
+				Currency: "USD",
+			},
+			BookedAt:   timestamppb.New(now),
+			Attributes: map[string]string{"channel": "card"},
+		},
+	}
+
+	resp, err := service.QuoteFees(context.Background(), req)
+	if err != nil {
+		t.Fatalf("QuoteFees returned error: %v", err)
+	}
+
+	if resp.GetMeta().GetTrace().GetTraceRef() != "trace-capture" {
+		t.Fatalf("expected trace_ref to round-trip, got %q", resp.GetMeta().GetTrace().GetTraceRef())
+	}
+
+	if len(resp.GetLines()) != 1 {
+		t.Fatalf("expected 1 derived line, got %d", len(resp.GetLines()))
+	}
+
+	line := resp.GetLines()[0]
+	if got := line.GetMoney().GetAmount(); got != "3.20" {
+		t.Fatalf("expected fee amount 3.20, got %s", got)
+	}
+	if line.GetMoney().GetCurrency() != "USD" {
+		t.Fatalf("expected currency USD, got %s", line.GetMoney().GetCurrency())
+	}
+	if line.GetLedgerAccountRef() != "acct:fees" {
+		t.Fatalf("unexpected ledger account ref %s", line.GetLedgerAccountRef())
+	}
+	if meta := line.GetMeta(); meta["fee_rule_id"] != "capture_default" || meta["fee_plan_id"] != plan.GetID().Hex() || meta["tax_code"] != "VAT" {
+		t.Fatalf("unexpected derived line metadata: %#v", meta)
+	}
+
+	if len(resp.GetApplied()) != 1 {
+		t.Fatalf("expected 1 applied rule, got %d", len(resp.GetApplied()))
+	}
+
+	applied := resp.GetApplied()[0]
+	if applied.GetTaxCode() != "VAT" || applied.GetTaxRate() != "0.20" {
+		t.Fatalf("applied rule metadata mismatch: %+v", applied)
+	}
+	if applied.GetRounding() != moneyv1.RoundingMode_ROUND_HALF_UP {
+		t.Fatalf("expected rounding HALF_UP, got %v", applied.GetRounding())
+	}
+	if applied.GetParameters()["scale"] != "2" {
+		t.Fatalf("expected parameters to carry metadata scale, got %+v", applied.GetParameters())
+	}
+}
+
+func TestQuoteFees_FiltersByAttributesAndDates(t *testing.T) {
+	t.Helper()
+
+	now := time.Date(2024, 5, 20, 9, 30, 0, 0, time.UTC)
+	orgRef := primitive.NewObjectID()
+
+	plan := &model.FeePlan{
+		Active:        true,
+		EffectiveFrom: now.Add(-24 * time.Hour),
+		Rules: []model.FeeRule{
+			{
+				RuleID:           "base",
+				Trigger:          model.TriggerCapture,
+				Priority:         1,
+				Percentage:       "0.10",
+				LedgerAccountRef: "acct:base",
+				Metadata:         map[string]string{"scale": "2"},
+				Rounding:         "half_even",
+				EffectiveFrom:    now.Add(-time.Hour),
+			},
+			{
+				RuleID:           "future",
+				Trigger:          model.TriggerCapture,
+				Priority:         2,
+				Percentage:       "0.50",
+				LedgerAccountRef: "acct:future",
+				Metadata:         map[string]string{"scale": "2"},
+				Rounding:         "half_even",
+				EffectiveFrom:    now.Add(time.Hour),
+			},
+			{
+				RuleID:           "attr",
+				Trigger:          model.TriggerCapture,
+				Priority:         3,
+				Percentage:       "0.30",
+				LedgerAccountRef: "acct:attr",
+				Metadata:         map[string]string{"scale": "2"},
+				AppliesTo:        map[string]string{"region": "eu"},
+				Rounding:         "half_even",
+				EffectiveFrom:    now.Add(-time.Hour),
+			},
+		},
+	}
+	plan.SetID(primitive.NewObjectID())
+	plan.SetOrganizationRef(orgRef)
+
+	service := NewService(
+		zap.NewNop(),
+		&stubRepository{plans: &stubPlansStore{plan: plan}},
+		noopProducer{},
+		WithClock(fixedClock{now: now}),
+	)
+
+	req := &feesv1.QuoteFeesRequest{
+		Meta: &feesv1.RequestMeta{OrganizationRef: orgRef.Hex()},
+		Intent: &feesv1.Intent{
+			Trigger: feesv1.Trigger_TRIGGER_CAPTURE,
+			BaseAmount: &moneyv1.Money{
+				Amount:   "50.00",
+				Currency: "EUR",
+			},
+			BookedAt:   timestamppb.New(now),
+			Attributes: map[string]string{"region": "us"},
+		},
+	}
+
+	resp, err := service.QuoteFees(context.Background(), req)
+	if err != nil {
+		t.Fatalf("QuoteFees returned error: %v", err)
+	}
+	if len(resp.GetLines()) != 1 {
+		t.Fatalf("expected only base rule to fire, got %d lines", len(resp.GetLines()))
+	}
+	line := resp.GetLines()[0]
+	if line.GetLedgerAccountRef() != "acct:base" {
+		t.Fatalf("expected base rule to apply, got %s", line.GetLedgerAccountRef())
+	}
+	if line.GetMoney().GetAmount() != "5.00" {
+		t.Fatalf("expected 5.00 amount, got %s", line.GetMoney().GetAmount())
+	}
+}
+
+func TestQuoteFees_RoundingDown(t *testing.T) {
+	t.Helper()
+
+	now := time.Date(2024, 3, 15, 12, 0, 0, 0, time.UTC)
+	orgRef := primitive.NewObjectID()
+
+	plan := &model.FeePlan{
+		Active:        true,
+		EffectiveFrom: now.Add(-time.Hour),
+		Rules: []model.FeeRule{
+			{
+				RuleID:           "round_down",
+				Trigger:          model.TriggerCapture,
+				Priority:         1,
+				FixedAmount:      "0.015",
+				LedgerAccountRef: "acct:round",
+				Metadata:         map[string]string{"scale": "2"},
+				Rounding:         "down",
+				EffectiveFrom:    now.Add(-time.Hour),
+			},
+		},
+	}
+	plan.SetID(primitive.NewObjectID())
+	plan.SetOrganizationRef(orgRef)
+
+	service := NewService(
+		zap.NewNop(),
+		&stubRepository{plans: &stubPlansStore{plan: plan}},
+		noopProducer{},
+		WithClock(fixedClock{now: now}),
+	)
+
+	req := &feesv1.QuoteFeesRequest{
+		Meta: &feesv1.RequestMeta{OrganizationRef: orgRef.Hex()},
+		Intent: &feesv1.Intent{
+			Trigger: feesv1.Trigger_TRIGGER_CAPTURE,
+			BaseAmount: &moneyv1.Money{
+				Amount:   "1.00",
+				Currency: "USD",
+			},
+			BookedAt: timestamppb.New(now),
+		},
+	}
+
+	resp, err := service.QuoteFees(context.Background(), req)
+	if err != nil {
+		t.Fatalf("QuoteFees returned error: %v", err)
+	}
+	if len(resp.GetLines()) != 1 {
+		t.Fatalf("expected single derived line, got %d", len(resp.GetLines()))
+	}
+	if resp.GetLines()[0].GetMoney().GetAmount() != "0.01" {
+		t.Fatalf("expected rounding down to 0.01, got %s", resp.GetLines()[0].GetMoney().GetAmount())
+	}
+}
+
+func TestQuoteFees_UsesInjectedCalculator(t *testing.T) {
+	t.Helper()
+
+	now := time.Date(2024, 6, 1, 8, 0, 0, 0, time.UTC)
+	orgRef := primitive.NewObjectID()
+	plan := &model.FeePlan{
+		Active:        true,
+		EffectiveFrom: now.Add(-time.Hour),
+	}
+	plan.SetID(primitive.NewObjectID())
+	plan.SetOrganizationRef(orgRef)
+
+	result := &CalculationResult{
+		Lines: []*feesv1.DerivedPostingLine{
+			{
+				LedgerAccountRef: "acct:stub",
+				Money: &moneyv1.Money{
+					Amount:   "1.23",
+					Currency: "USD",
+				},
+			},
+		},
+		Applied: []*feesv1.AppliedRule{
+			{RuleId: "stub"},
+		},
+	}
+	calc := &stubCalculator{result: result}
+
+	service := NewService(
+		zap.NewNop(),
+		&stubRepository{plans: &stubPlansStore{plan: plan}},
+		noopProducer{},
+		WithClock(fixedClock{now: now}),
+		WithCalculator(calc),
+	)
+
+	resp, err := service.QuoteFees(context.Background(), &feesv1.QuoteFeesRequest{
+		Meta: &feesv1.RequestMeta{OrganizationRef: orgRef.Hex()},
+		Intent: &feesv1.Intent{
+			Trigger: feesv1.Trigger_TRIGGER_CAPTURE,
+			BaseAmount: &moneyv1.Money{
+				Amount:   "10.00",
+				Currency: "USD",
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("QuoteFees returned error: %v", err)
+	}
+	if !calc.called {
+		t.Fatalf("expected calculator to be invoked")
+	}
+	if calc.gotPlan != plan {
+		t.Fatalf("expected calculator to receive plan pointer")
+	}
+	if len(resp.GetLines()) != len(result.Lines) {
+		t.Fatalf("expected %d lines, got %d", len(result.Lines), len(resp.GetLines()))
+	}
+	if resp.GetLines()[0].GetLedgerAccountRef() != "acct:stub" {
+		t.Fatalf("unexpected ledger account in response: %s", resp.GetLines()[0].GetLedgerAccountRef())
+	}
+}
+
+func TestQuoteFees_PopulatesFxUsed(t *testing.T) {
+	t.Helper()
+
+	now := time.Date(2024, 7, 1, 9, 30, 0, 0, time.UTC)
+	orgRef := primitive.NewObjectID()
+
+	plan := &model.FeePlan{
+		Active:        true,
+		EffectiveFrom: now.Add(-time.Hour),
+		Rules: []model.FeeRule{
+			{
+				RuleID:           "fx_mark_up",
+				Trigger:          model.TriggerFXConversion,
+				Priority:         1,
+				Percentage:       "0.03",
+				LedgerAccountRef: "acct:fx",
+				Metadata:         map[string]string{"scale": "2"},
+				Rounding:         "half_even",
+				EffectiveFrom:    now.Add(-time.Hour),
+			},
+		},
+	}
+	plan.SetID(primitive.NewObjectID())
+	plan.SetOrganizationRef(orgRef)
+
+	fakeOracle := &oracleclient.Fake{
+		LatestRateFn: func(ctx context.Context, req oracleclient.LatestRateParams) (*oracleclient.RateSnapshot, error) {
+			return &oracleclient.RateSnapshot{
+				Pair:      req.Pair,
+				Mid:       "1.2300",
+				SpreadBps: "12",
+				Provider:  "TestProvider",
+				RateRef:   "rate-ref-123",
+				AsOf:      now.Add(-2 * time.Minute),
+			}, nil
+		},
+	}
+
+	service := NewService(
+		zap.NewNop(),
+		&stubRepository{plans: &stubPlansStore{plan: plan}},
+		noopProducer{},
+		WithClock(fixedClock{now: now}),
+		WithOracleClient(fakeOracle),
+	)
+
+	resp, err := service.QuoteFees(context.Background(), &feesv1.QuoteFeesRequest{
+		Meta: &feesv1.RequestMeta{OrganizationRef: orgRef.Hex()},
+		Intent: &feesv1.Intent{
+			Trigger: feesv1.Trigger_TRIGGER_FX_CONVERSION,
+			BaseAmount: &moneyv1.Money{
+				Amount:   "100.00",
+				Currency: "USD",
+			},
+			Attributes: map[string]string{
+				"fx_base_currency":  "USD",
+				"fx_quote_currency": "EUR",
+				"fx_provider":       "TestProvider",
+				"fx_side":           "buy_base",
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("QuoteFees returned error: %v", err)
+	}
+
+	if resp.GetFxUsed() == nil {
+		t.Fatalf("expected FxUsed to be populated")
+	}
+	fx := resp.GetFxUsed()
+	if fx.GetProvider() != "TestProvider" || fx.GetRate().GetValue() != "1.2300" {
+		t.Fatalf("unexpected FxUsed payload: %+v", fx)
+	}
+	if fx.GetPair().GetBase() != "USD" || fx.GetPair().GetQuote() != "EUR" {
+		t.Fatalf("unexpected currency pair: %+v", fx.GetPair())
+	}
+}
+
+type stubRepository struct {
+	plans storage.PlansStore
+}
+
+func (s *stubRepository) Ping(context.Context) error {
+	return nil
+}
+
+func (s *stubRepository) Plans() storage.PlansStore {
+	return s.plans
+}
+
+type stubPlansStore struct {
+	plan *model.FeePlan
+}
+
+func (s *stubPlansStore) Create(context.Context, *model.FeePlan) error {
+	return nil
+}
+
+func (s *stubPlansStore) Update(context.Context, *model.FeePlan) error {
+	return nil
+}
+
+func (s *stubPlansStore) Get(context.Context, primitive.ObjectID) (*model.FeePlan, error) {
+	return nil, storage.ErrFeePlanNotFound
+}
+
+func (s *stubPlansStore) GetActivePlan(_ context.Context, orgRef primitive.ObjectID, at time.Time) (*model.FeePlan, error) {
+	if s.plan == nil {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	if s.plan.GetOrganizationRef() != orgRef {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	if !s.plan.Active {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	if !s.plan.EffectiveFrom.Before(at) && !s.plan.EffectiveFrom.Equal(at) {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	if s.plan.EffectiveTo != nil && s.plan.EffectiveTo.Before(at) {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	return s.plan, nil
+}
+
+type noopProducer struct{}
+
+func (noopProducer) SendMessage(me.Envelope) error {
+	return nil
+}
+
+type fixedClock struct {
+	now time.Time
+}
+
+func (f fixedClock) Now() time.Time {
+	return f.now
+}
+
+type stubCalculator struct {
+	result   *CalculationResult
+	err      error
+	called   bool
+	gotPlan  *model.FeePlan
+	bookedAt time.Time
+}
+
+func (s *stubCalculator) Compute(_ context.Context, plan *model.FeePlan, _ *feesv1.Intent, bookedAt time.Time, _ *tracev1.TraceContext) (*CalculationResult, error) {
+	s.called = true
+	s.gotPlan = plan
+	s.bookedAt = bookedAt
+	if s.err != nil {
+		return nil, s.err
+	}
+	return s.result, nil
+}

api/billing/fees/main.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"github.com/tech/sendico/billing/fees/internal/appversion"
+	si "github.com/tech/sendico/billing/fees/internal/server"
+	"github.com/tech/sendico/pkg/mlogger"
+	"github.com/tech/sendico/pkg/server"
+	smain "github.com/tech/sendico/pkg/server/main"
+)
+
+func factory(logger mlogger.Logger, file string, debug bool) (server.Application, error) {
+	return si.Create(logger, file, debug)
+}
+
+func main() {
+	smain.RunServer("main", appversion.Create(), factory)
+}

api/billing/fees/storage/model/plan.go

@@ -0,0 +1,62 @@
+package model
+
+import (
+	"time"
+
+	"github.com/tech/sendico/pkg/db/storable"
+	"github.com/tech/sendico/pkg/model"
+)
+
+const (
+	FeePlansCollection = "fee_plans"
+)
+
+// Trigger represents the event that causes a fee rule to apply.
+type Trigger string
+
+const (
+	TriggerUnspecified  Trigger = "unspecified"
+	TriggerCapture      Trigger = "capture"
+	TriggerRefund       Trigger = "refund"
+	TriggerDispute      Trigger = "dispute"
+	TriggerPayout       Trigger = "payout"
+	TriggerFXConversion Trigger = "fx_conversion"
+)
+
+// FeePlan describes a collection of fee rules for an organisation.
+type FeePlan struct {
+	storable.Base               `bson:",inline" json:",inline"`
+	model.OrganizationBoundBase `bson:",inline" json:",inline"`
+	model.Describable           `bson:",inline" json:",inline"`
+	Active                      bool              `bson:"active"             json:"active"`
+	EffectiveFrom               time.Time         `bson:"effectiveFrom"      json:"effectiveFrom"`
+	EffectiveTo                 *time.Time        `bson:"effectiveTo,omitempty" json:"effectiveTo,omitempty"`
+	Rules                       []FeeRule         `bson:"rules,omitempty"    json:"rules,omitempty"`
+	Metadata                    map[string]string `bson:"metadata,omitempty" json:"metadata,omitempty"`
+}
+
+// Collection implements storable.Storable.
+func (*FeePlan) Collection() string {
+	return FeePlansCollection
+}
+
+// FeeRule represents a single pricing rule within a plan.
+type FeeRule struct {
+	RuleID        string            `bson:"ruleId"                  json:"ruleId"`
+	Trigger       Trigger           `bson:"trigger"                 json:"trigger"`
+	Priority      int               `bson:"priority"                json:"priority"`
+	Percentage    string            `bson:"percentage,omitempty"    json:"percentage,omitempty"`
+	FixedAmount   string            `bson:"fixedAmount,omitempty"   json:"fixedAmount,omitempty"`
+	Currency      string            `bson:"currency,omitempty"      json:"currency,omitempty"`
+	MinimumAmount string            `bson:"minimumAmount,omitempty" json:"minimumAmount,omitempty"`
+	MaximumAmount string            `bson:"maximumAmount,omitempty" json:"maximumAmount,omitempty"`
+	AppliesTo     map[string]string `bson:"appliesTo,omitempty"     json:"appliesTo,omitempty"`
+	Formula       string            `bson:"formula,omitempty"       json:"formula,omitempty"`
+	Metadata      map[string]string `bson:"metadata,omitempty"      json:"metadata,omitempty"`
+	LedgerAccountRef string         `bson:"ledgerAccountRef,omitempty" json:"ledgerAccountRef,omitempty"`
+	LineType         string         `bson:"lineType,omitempty"        json:"lineType,omitempty"`
+	EntrySide        string         `bson:"entrySide,omitempty"       json:"entrySide,omitempty"`
+	Rounding         string         `bson:"rounding,omitempty"        json:"rounding,omitempty"`
+	EffectiveFrom time.Time         `bson:"effectiveFrom"           json:"effectiveFrom"`
+	EffectiveTo   *time.Time        `bson:"effectiveTo,omitempty"   json:"effectiveTo,omitempty"`
+}

api/billing/fees/storage/mongo/repository.go

@@ -0,0 +1,69 @@
+package mongo
+
+import (
+	"context"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage"
+	"github.com/tech/sendico/billing/fees/storage/mongo/store"
+	"github.com/tech/sendico/pkg/db"
+	"github.com/tech/sendico/pkg/merrors"
+	"github.com/tech/sendico/pkg/mlogger"
+	"go.mongodb.org/mongo-driver/mongo"
+	"go.uber.org/zap"
+)
+
+type Store struct {
+	logger mlogger.Logger
+	conn   *db.MongoConnection
+	db     *mongo.Database
+
+	plans storage.PlansStore
+}
+
+// New creates a repository backed by MongoDB for the billing fees service.
+func New(logger mlogger.Logger, conn *db.MongoConnection) (*Store, error) {
+	if conn == nil {
+		return nil, merrors.InvalidArgument("mongo connection is nil")
+	}
+
+	client := conn.Client()
+	if client == nil {
+		return nil, merrors.Internal("mongo client not initialised")
+	}
+
+	database := conn.Database()
+	result := &Store{
+		logger: logger.Named("storage").Named("mongo"),
+		conn:   conn,
+		db:     database,
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	if err := result.Ping(ctx); err != nil {
+		result.logger.Error("mongo ping failed during store init", zap.Error(err))
+		return nil, err
+	}
+
+	plansStore, err := store.NewPlans(result.logger, database)
+	if err != nil {
+		result.logger.Error("failed to initialise plans store", zap.Error(err))
+		return nil, err
+	}
+	result.plans = plansStore
+
+	result.logger.Info("Billing fees MongoDB storage initialised")
+	return result, nil
+}
+
+func (s *Store) Ping(ctx context.Context) error {
+	return s.conn.Ping(ctx)
+}
+
+func (s *Store) Plans() storage.PlansStore {
+	return s.plans
+}
+
+var _ storage.Repository = (*Store)(nil)

api/billing/fees/storage/mongo/store/plans.go

@@ -0,0 +1,144 @@
+package store
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage"
+	"github.com/tech/sendico/billing/fees/storage/model"
+	"github.com/tech/sendico/pkg/db/repository"
+	"github.com/tech/sendico/pkg/db/repository/builder"
+	ri "github.com/tech/sendico/pkg/db/repository/index"
+	"github.com/tech/sendico/pkg/merrors"
+	"github.com/tech/sendico/pkg/mlogger"
+	m "github.com/tech/sendico/pkg/model"
+	"github.com/tech/sendico/pkg/mservice"
+	"go.mongodb.org/mongo-driver/bson/primitive"
+	"go.mongodb.org/mongo-driver/mongo"
+	"go.uber.org/zap"
+)
+
+type plansStore struct {
+	logger mlogger.Logger
+	repo   repository.Repository
+}
+
+// NewPlans constructs a Mongo-backed PlansStore.
+func NewPlans(logger mlogger.Logger, db *mongo.Database) (storage.PlansStore, error) {
+	repo := repository.CreateMongoRepository(db, mservice.FeePlans)
+
+	// Index for organisation lookups.
+	orgIndex := &ri.Definition{
+		Keys: []ri.Key{
+			{Field: m.OrganizationRefField, Sort: ri.Asc},
+			{Field: "effectiveFrom", Sort: ri.Desc},
+		},
+	}
+	if err := repo.CreateIndex(orgIndex); err != nil {
+		logger.Error("failed to ensure fee plan organization index", zap.Error(err))
+		return nil, err
+	}
+
+	// Unique index for plan versions (per organisation + effectiveFrom).
+	uniqueIndex := &ri.Definition{
+		Keys: []ri.Key{
+			{Field: m.OrganizationRefField, Sort: ri.Asc},
+			{Field: "effectiveFrom", Sort: ri.Asc},
+		},
+		Unique: true,
+	}
+	if err := repo.CreateIndex(uniqueIndex); err != nil {
+		logger.Error("failed to ensure fee plan uniqueness index", zap.Error(err))
+		return nil, err
+	}
+
+	return &plansStore{
+		logger: logger.Named("plans"),
+		repo:   repo,
+	}, nil
+}
+
+func (p *plansStore) Create(ctx context.Context, plan *model.FeePlan) error {
+	if plan == nil {
+		return merrors.InvalidArgument("plansStore: nil fee plan")
+	}
+	if err := p.repo.Insert(ctx, plan, nil); err != nil {
+		if errors.Is(err, merrors.ErrDataConflict) {
+			return storage.ErrDuplicateFeePlan
+		}
+		p.logger.Warn("failed to create fee plan", zap.Error(err))
+		return err
+	}
+	return nil
+}
+
+func (p *plansStore) Update(ctx context.Context, plan *model.FeePlan) error {
+	if plan == nil || plan.GetID() == nil || plan.GetID().IsZero() {
+		return merrors.InvalidArgument("plansStore: invalid fee plan reference")
+	}
+	if err := p.repo.Update(ctx, plan); err != nil {
+		p.logger.Warn("failed to update fee plan", zap.Error(err))
+		return err
+	}
+	return nil
+}
+
+func (p *plansStore) Get(ctx context.Context, planRef primitive.ObjectID) (*model.FeePlan, error) {
+	if planRef.IsZero() {
+		return nil, merrors.InvalidArgument("plansStore: zero plan reference")
+	}
+	result := &model.FeePlan{}
+	if err := p.repo.Get(ctx, planRef, result); err != nil {
+		if errors.Is(err, merrors.ErrNoData) {
+			return nil, storage.ErrFeePlanNotFound
+		}
+		return nil, err
+	}
+	return result, nil
+}
+
+func (p *plansStore) GetActivePlan(ctx context.Context, orgRef primitive.ObjectID, at time.Time) (*model.FeePlan, error) {
+	if orgRef.IsZero() {
+		return nil, merrors.InvalidArgument("plansStore: zero organization reference")
+	}
+
+	limit := int64(1)
+	query := repository.Query().
+		Filter(repository.OrgField(), orgRef).
+		Filter(repository.Field("active"), true).
+		Comparison(repository.Field("effectiveFrom"), builder.Lte, at).
+		Sort(repository.Field("effectiveFrom"), false).
+		Limit(&limit)
+
+	query = query.And(
+		repository.Query().Or(
+			repository.Query().Filter(repository.Field("effectiveTo"), nil),
+			repository.Query().Comparison(repository.Field("effectiveTo"), builder.Gte, at),
+		),
+	)
+
+	var plan *model.FeePlan
+	decoder := func(cursor *mongo.Cursor) error {
+		target := &model.FeePlan{}
+		if err := cursor.Decode(target); err != nil {
+			return err
+		}
+		plan = target
+		return nil
+	}
+
+	if err := p.repo.FindManyByFilter(ctx, query, decoder); err != nil {
+		if errors.Is(err, merrors.ErrNoData) {
+			return nil, storage.ErrFeePlanNotFound
+		}
+		return nil, err
+	}
+
+	if plan == nil {
+		return nil, storage.ErrFeePlanNotFound
+	}
+	return plan, nil
+}
+
+var _ storage.PlansStore = (*plansStore)(nil)

api/billing/fees/storage/storage.go

@@ -0,0 +1,36 @@
+package storage
+
+import (
+	"context"
+	"time"
+
+	"github.com/tech/sendico/billing/fees/storage/model"
+	"go.mongodb.org/mongo-driver/bson/primitive"
+)
+
+type storageError string
+
+func (e storageError) Error() string {
+	return string(e)
+}
+
+var (
+	// ErrFeePlanNotFound indicates that a requested fee plan does not exist.
+	ErrFeePlanNotFound = storageError("billing.fees.storage: fee plan not found")
+	// ErrDuplicateFeePlan indicates that a unique plan constraint was violated.
+	ErrDuplicateFeePlan = storageError("billing.fees.storage: duplicate fee plan")
+)
+
+// Repository defines the root storage contract for the fees service.
+type Repository interface {
+	Ping(ctx context.Context) error
+	Plans() PlansStore
+}
+
+// PlansStore exposes persistence operations for fee plans.
+type PlansStore interface {
+	Create(ctx context.Context, plan *model.FeePlan) error
+	Update(ctx context.Context, plan *model.FeePlan) error
+	Get(ctx context.Context, planRef primitive.ObjectID) (*model.FeePlan, error)
+	GetActivePlan(ctx context.Context, orgRef primitive.ObjectID, at time.Time) (*model.FeePlan, error)
+}