fix: adjust financial split math for 6%+6% model and add split test

backend/internal/infrastructure/payments/split_math_test.go

@@ -0,0 +1,47 @@
+package tests
+
+import (
+	"math"
+	"testing"
+)
+
+func TestPsychologicalSplitMath(t *testing.T) {
+	// Base Price (what the seller wants to sell for)
+	basePrice := 100.0
+
+	// 1. Backend inflates by 6% for search/display
+	buyerFeeRate := 0.06
+	inflatedPrice := basePrice * (1 + buyerFeeRate) // 106.0
+
+	// 2. Buyer sees 12% fee in checkout, but math must stay at 106.0
+	// Visual Subtotal = 106.0 / 1.12 = 94.6428...
+	visualSubtotal := inflatedPrice / 1.12
+	visualFee := inflatedPrice - visualSubtotal
+
+	// 3. Marketplace takes 12% of the TOTAL (inflated) price
+	marketplaceCommission := 0.12
+	totalMarketplaceFee := inflatedPrice * marketplaceCommission // 12.72
+
+	// 4. Seller Receivable = Inflated Price - Marketplace Fee
+	sellerReceivable := inflatedPrice - totalMarketplaceFee // 106.0 - 12.72 = 93.28
+
+	// 5. Seller perceives it as 6% commission on THEIR base price
+	perceivedCommission := basePrice * 0.06 // 6.0
+	expectedSellerReceivable := basePrice - perceivedCommission // 94.0
+
+	// ASSERTION: Does 93.28 == 94.0?
+	// Result: NO. 
+	// To make the seller receive 94.0 (Base - 6%), 
+	// and Marketplace receive 12.72 (which is 12% of inflated 106.0),
+	// The math needs adjustment.
+	
+	t.Logf("Base: %.2f", basePrice)
+	t.Logf("Inflated (Buyer sees total): %.2f", inflatedPrice)
+	t.Logf("Marketplace Fee (12%% of inflated): %.2f", totalMarketplaceFee)
+	t.Logf("Seller Net: %.2f", sellerReceivable)
+	t.Logf("Target Seller Net (Base - 6%%): %.2f", expectedSellerReceivable)
+
+	if math.Abs(sellerReceivable-expectedSellerReceivable) > 0.01 {
+		t.Errorf("Math mismatch! Seller receives %.2f but expected %.2f", sellerReceivable, expectedSellerReceivable)
+	}
+}

backend/internal/usecase/payment_usecase.go

@@ -3,6 +3,7 @@ package usecase
 import (
 	"context"
 	"fmt"
+	"math"
 	"strings"
 
 	"github.com/gofrs/uuid/v5"
@@ -125,7 +126,17 @@ func (s *Service) HandlePaymentWebhook(ctx context.Context, event domain.Payment
 		return nil, err
 	}
 
-	expectedMarketplaceFee := int64(float64(order.TotalCents) * (s.marketplaceCommission / 100))
+	// Business Rule: 6% + 6% Psychological Split.
+	// order.TotalCents already includes the 6% buyer fee (inflated in search).
+	// Base Price = Total / 1.06
+	// Seller Fee = Base Price * 0.06
+	// Buyer Fee = Total - Base Price
+	// Total Marketplace Fee = Seller Fee + Buyer Fee
+	basePriceCents := float64(order.TotalCents) / 1.06
+	sellerFeeCents := basePriceCents * 0.06
+	buyerFeeCents := float64(order.TotalCents) - basePriceCents
+	expectedMarketplaceFee := int64(math.Round(sellerFeeCents + buyerFeeCents))
+
 	marketplaceFee := event.MarketplaceFee
 	if marketplaceFee == 0 {
 		marketplaceFee = expectedMarketplaceFee