diff --git a/checkout.go b/checkout.go
index b4e7a3b..23d0b1a 100644
--- a/checkout.go
+++ b/checkout.go
@@ -1,6 +1,6 @@
 package main
 
 func checkout(receipt *Receipt) ledger {
-	unitTotal := receipt.UnitPrice - receipt.Discount
-	return ledger{Total: unitTotal * receipt.Quantity, Writes: 1}
+	subtotal := receipt.UnitPrice * receipt.Quantity
+	return ledger{Total: subtotal - receipt.Discount, Writes: 1}
 }
diff --git a/promotion_once_regression_test.go b/promotion_once_regression_test.go
new file mode 100644
index 0000000..5115001
--- /dev/null
+++ b/promotion_once_regression_test.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"context"
+	"net/http"
+	"testing"
+)
+
+func TestWelcome100OncePerOrderPersistedLedger(t *testing.T) {
+	s, m := testService()
+	request := CheckoutRequest{OrderID: "welcome-once-five", SKU: "COFFEE-250", Quantity: 5, PromotionCode: "WELCOME100"}
+	response, receipt := submit(t, s, request)
+	if response.Code != http.StatusCreated {
+		t.Fatalf("create status: got %d want 201", response.Code)
+	}
+	stored, err := m.ReadOrder(context.Background(), request.OrderID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Pinned README: 1000 cents * 5 items - 100 cents once = 4900 cents.
+	if stored.Ledger.Total != 4900 || stored.Ledger.Writes != 1 {
+		t.Fatalf("persisted once-per-order ledger: got %+v want total=4900 writes=1", stored.Ledger)
+	}
+	if stored.Request != request || stored.Receipt.UnitPrice != 1000 || stored.Receipt.Discount != 100 || stored.Receipt.Currency != "USD" {
+		t.Fatalf("accepted request or pricing snapshot changed: %+v", stored)
+	}
+	if receipt.LedgerTotal == nil || *receipt.LedgerTotal != 4900 || receipt.LedgerWrites == nil || *receipt.LedgerWrites != 1 {
+		t.Fatalf("create receipt disagrees with persisted ledger: %+v", receipt)
+	}
+	response, replay := submit(t, s, request)
+	if response.Code != http.StatusOK || response.Header().Get("Idempotency-Replayed") != "true" || replay.LedgerTotal == nil || *replay.LedgerTotal != 4900 || replay.LedgerWrites == nil || *replay.LedgerWrites != 1 {
+		t.Fatalf("retry changed accepted outcome: status=%d receipt=%+v", response.Code, replay)
+	}
+	after, err := m.ReadOrder(context.Background(), request.OrderID)
+	if err != nil || len(m.orders) != 1 || after.Ledger != stored.Ledger || after.Request != stored.Request || after.RequestSHA256 != stored.RequestSHA256 {
+		t.Fatalf("retry mutated stored order: err=%v order=%+v", err, after)
+	}
+}