fix: remove naked flat fee support

app/common/billing.go

@@ -71,7 +71,7 @@ func BillingService(
 		return nil, err
 	}
 
-	handler, err := NewBillingSubscriptionHandler(logger, subscriptionServices, service, billingAdapter, tracer, fsConfig)
+	handler, err := NewBillingSubscriptionHandler(logger, subscriptionServices, service, billingAdapter, tracer)
 	if err != nil {
 		return nil, err
 	}
@@ -119,16 +119,11 @@ func NewBillingSubscriptionReconciler(logger *slog.Logger, subsServices Subscrip
 	})
 }
 
-func NewBillingSubscriptionHandler(logger *slog.Logger, subsServices SubscriptionServiceWithWorkflow, billingService billing.Service, billingAdapter billing.Adapter, tracer trace.Tracer, config config.BillingFeatureSwitchesConfiguration) (*billingworkersubscription.Handler, error) {
-	featureFlags := billingworkersubscription.FeatureFlags{
-		UseUsageBasedFlatFeeLines: config.UseUsageBasedFlatFeeLines,
-	}
-
+func NewBillingSubscriptionHandler(logger *slog.Logger, subsServices SubscriptionServiceWithWorkflow, billingService billing.Service, billingAdapter billing.Adapter, tracer trace.Tracer) (*billingworkersubscription.Handler, error) {
 	return billingworkersubscription.New(billingworkersubscription.Config{
 		SubscriptionService: subsServices.Service,
 		BillingService:      billingService,
 		TxCreator:           billingAdapter,
-		FeatureFlags:        featureFlags,
 		Logger:              logger,
 		Tracer:              tracer,
 	})

app/config/billing.go

@@ -34,8 +34,7 @@ func (c BillingConfiguration) Validate() error {
 }
 
 type BillingFeatureSwitchesConfiguration struct {
-	NamespaceLockdown         []string
-	UseUsageBasedFlatFeeLines bool
+	NamespaceLockdown []string
 }
 
 func (c BillingFeatureSwitchesConfiguration) Validate() error {

openmeter/billing/httpdriver/invoiceline.go

@@ -70,12 +70,10 @@ func (h *handler) CreatePendingLine() CreatePendingLineHandler {
 				}
 			}
 
-			if h.featureSwitches.UseUsageBasedFlatFeeLines {
-				for _, line := range lineEntities {
-					if line.Type == billing.InvoiceLineTypeFee {
-						return CreatePendingLineRequest{}, billing.ValidationError{
-							Err: fmt.Errorf("creating flat fee lines is not supported, please use usage based lines instead"),
-						}
+			for _, line := range lineEntities {
+				if line.Type == billing.InvoiceLineTypeFee {
+					return CreatePendingLineRequest{}, billing.ValidationError{
+						Err: fmt.Errorf("creating flat fee lines is not supported, please use usage based lines instead"),
 					}
 				}
 			}
@@ -713,7 +711,7 @@ func (h *handler) mergeInvoiceLinesFromAPI(ctx context.Context, invoice *billing
 				return billing.LineChildren{}, fmt.Errorf("failed to create new line: %w", err)
 			}
 
-			if h.featureSwitches.UseUsageBasedFlatFeeLines && newLine.Type == billing.InvoiceLineTypeFee {
+			if newLine.Type == billing.InvoiceLineTypeFee {
 				return billing.LineChildren{}, billing.ValidationError{
 					Err: fmt.Errorf("creating flat fee lines is not supported, please use usage based lines instead"),
 				}
@@ -739,7 +737,7 @@ func (h *handler) mergeInvoiceLinesFromAPI(ctx context.Context, invoice *billing
 			return billing.LineChildren{}, fmt.Errorf("failed to merge line: %w", err)
 		}
 
-		if h.featureSwitches.UseUsageBasedFlatFeeLines && changed && mergedLine.Type == billing.InvoiceLineTypeFee {
+		if changed && mergedLine.Type == billing.InvoiceLineTypeFee {
 			return billing.LineChildren{}, billing.ValidationError{
 				Err: fmt.Errorf("updating flat fee lines is not supported, please use usage based lines instead"),
 			}

openmeter/billing/worker/subscription/feehelper.go

@@ -17,10 +17,6 @@ func isFlatFee(line *billing.Line) bool {
 		return false
 	}
 
-	if line.Type == billing.InvoiceLineTypeFee {
-		return true
-	}
-
 	if line.Type == billing.InvoiceLineTypeUsageBased &&
 		line.UsageBased != nil &&
 		line.UsageBased.Price != nil &&
@@ -36,58 +32,43 @@ func getFlatFeePerUnitAmount(line *billing.Line) (alpacadecimal.Decimal, error)
 		return alpacadecimal.Zero, fmt.Errorf("line is nil")
 	}
 
-	switch line.Type {
-	case billing.InvoiceLineTypeFee:
-		if line.FlatFee == nil {
-			return alpacadecimal.Zero, fmt.Errorf("line misses flat fee metadata")
-		}
-
-		return line.FlatFee.PerUnitAmount, nil
-	case billing.InvoiceLineTypeUsageBased:
-		if line.UsageBased == nil || line.UsageBased.Price == nil {
-			return alpacadecimal.Zero, fmt.Errorf("line misses usage based metadata")
-		}
-
-		flatPrice, err := line.UsageBased.Price.AsFlat()
-		if err != nil {
-			return alpacadecimal.Zero, err
-		}
-
-		return flatPrice.Amount, nil
-	default:
-		return alpacadecimal.Zero, fmt.Errorf("line is not a (flat or usage based) fee line")
+	if line.Type != billing.InvoiceLineTypeUsageBased {
+		return alpacadecimal.Zero, fmt.Errorf("line is not a usage based line")
+	}
+
+	if line.UsageBased == nil || line.UsageBased.Price == nil {
+		return alpacadecimal.Zero, fmt.Errorf("line misses usage based metadata")
+	}
+
+	flatPrice, err := line.UsageBased.Price.AsFlat()
+	if err != nil {
+		return alpacadecimal.Zero, err
 	}
+
+	return flatPrice.Amount, nil
 }
 
 func setFlatFeePerUnitAmount(line *billing.Line, perUnitAmount alpacadecimal.Decimal) error {
 	if line == nil {
 		return fmt.Errorf("line is nil")
 	}
 
-	switch line.Type {
-	case billing.InvoiceLineTypeFee:
-		if line.FlatFee == nil {
-			return fmt.Errorf("line misses flat fee metadata")
-		}
-
-		line.FlatFee.PerUnitAmount = perUnitAmount
-		return nil
-	case billing.InvoiceLineTypeUsageBased:
-		if line.UsageBased == nil || line.UsageBased.Price == nil {
-			return fmt.Errorf("line misses usage based metadata")
-		}
-
-		flatPrice, err := line.UsageBased.Price.AsFlat()
-		if err != nil {
-			return err
-		}
-
-		flatPrice.Amount = perUnitAmount
-		line.UsageBased.Price = productcatalog.NewPriceFrom(flatPrice)
-		return nil
-	default:
-		return fmt.Errorf("line is not a (flat or usage based) fee line")
+	if line.Type != billing.InvoiceLineTypeUsageBased {
+		return fmt.Errorf("line is not a usage based line")
+	}
+
+	if line.UsageBased == nil || line.UsageBased.Price == nil {
+		return fmt.Errorf("line misses usage based metadata")
 	}
+
+	flatPrice, err := line.UsageBased.Price.AsFlat()
+	if err != nil {
+		return err
+	}
+
+	flatPrice.Amount = perUnitAmount
+	line.UsageBased.Price = productcatalog.NewPriceFrom(flatPrice)
+	return nil
 }
 
 type typeWithEqual[T any] interface {

openmeter/billing/worker/subscription/sync.go

@@ -9,7 +9,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/alpacahq/alpacadecimal"
 	"github.com/samber/lo"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/trace"
@@ -32,7 +31,6 @@ const (
 type FeatureFlags struct {
 	EnableFlatFeeInAdvanceProrating bool
 	EnableFlatFeeInArrearsProrating bool
-	UseUsageBasedFlatFeeLines       bool
 }
 
 type Config struct {
@@ -617,20 +615,13 @@ func (h *Handler) lineFromSubscritionRateCard(subs subscription.SubscriptionView
 			return nil, nil
 		}
 
-		if !h.featureFlags.UseUsageBasedFlatFeeLines {
-			line.Type = billing.InvoiceLineTypeFee
-			line.FlatFee = &billing.FlatFeeLine{
-				PerUnitAmount: perUnitAmount,
-				Quantity:      alpacadecimal.NewFromInt(1),
-				PaymentTerm:   price.PaymentTerm,
-				Category:      billing.FlatFeeCategoryRegular,
-			}
-		} else {
-			line.Type = billing.InvoiceLineTypeUsageBased
-			line.UsageBased = &billing.UsageBasedLine{
-				Price:      item.SubscriptionItem.RateCard.AsMeta().Price,
-				FeatureKey: lo.FromPtr(item.SubscriptionItem.RateCard.AsMeta().FeatureKey),
-			}
+		line.Type = billing.InvoiceLineTypeUsageBased
+		line.UsageBased = &billing.UsageBasedLine{
+			Price: productcatalog.NewPriceFrom(productcatalog.FlatPrice{
+				Amount:      perUnitAmount,
+				PaymentTerm: price.PaymentTerm,
+			}),
+			FeatureKey: lo.FromPtr(item.SubscriptionItem.RateCard.AsMeta().FeatureKey),
 		}
 
 	default: