diff --git a/apps/backend/src/lib/payments.test.tsx b/apps/backend/src/lib/payments.test.tsx
index 4106999f92..10a0c7b2fa 100644
--- a/apps/backend/src/lib/payments.test.tsx
+++ b/apps/backend/src/lib/payments.test.tsx
@@ -842,7 +842,7 @@ describe('validatePurchaseSession - one-time purchase rules', () => {
       },
       priceId: 'price-any',
       quantity: 1,
-    })).rejects.toThrowError('Customer already has a one-time purchase for this product');
+    })).rejects.toThrowError('Customer already has purchased this product; this product is not stackable');
   });
 
   it('blocks one-time purchase when another one exists in the same group', async () => {
@@ -912,6 +912,157 @@ describe('validatePurchaseSession - one-time purchase rules', () => {
     expect(res.catalogId).toBe('g1');
     expect(res.conflictingCatalogSubscriptions.length).toBe(0);
   });
+
+  it('allows duplicate one-time purchase for same productId when product is stackable', async () => {
+    const tenancy = createMockTenancy({ items: {}, products: {}, catalogs: {} });
+    const prisma = createMockPrisma({
+      oneTimePurchase: {
+        findMany: async () => [
+          { productId: 'product-stackable', product: { catalogId: undefined }, quantity: 1, createdAt: new Date('2025-01-01T00:00:00.000Z') },
+        ],
+      },
+      subscription: { findMany: async () => [] },
+    } as any);
+
+    const res = await validatePurchaseSession({
+      prisma,
+      tenancy,
+      codeData: {
+        tenancyId: tenancy.id,
+        customerId: 'cust-1',
+        productId: 'product-stackable',
+        product: {
+          displayName: 'Stackable Product',
+          catalogId: undefined,
+          customerType: 'custom',
+          freeTrial: undefined,
+          serverOnly: false,
+          stackable: true,
+          prices: 'include-by-default',
+          includedItems: {},
+          isAddOnTo: false,
+        },
+      },
+      priceId: 'price-any',
+      quantity: 2,
+    });
+
+    expect(res.catalogId).toBeUndefined();
+    expect(res.conflictingCatalogSubscriptions.length).toBe(0);
+  });
+
+  it('blocks when subscription for same product exists and product is not stackable', async () => {
+    const tenancy = createMockTenancy({
+      items: {},
+      catalogs: {},
+      products: {
+        'product-sub': {
+          displayName: 'Non-stackable Offer',
+          catalogId: undefined,
+          customerType: 'custom',
+          freeTrial: undefined,
+          serverOnly: false,
+          stackable: false,
+          prices: {},
+          includedItems: {},
+          isAddOnTo: false,
+        },
+      },
+    });
+    const prisma = createMockPrisma({
+      oneTimePurchase: { findMany: async () => [] },
+      subscription: {
+        findMany: async () => [{
+          productId: 'product-sub',
+          currentPeriodStart: new Date('2025-02-01T00:00:00.000Z'),
+          currentPeriodEnd: new Date('2025-03-01T00:00:00.000Z'),
+          quantity: 1,
+          status: 'active',
+        }],
+      },
+    } as any);
+
+    await expect(validatePurchaseSession({
+      prisma,
+      tenancy,
+      codeData: {
+        tenancyId: tenancy.id,
+        customerId: 'cust-1',
+        productId: 'product-sub',
+        product: {
+          displayName: 'Non-stackable Offer',
+          catalogId: undefined,
+          customerType: 'custom',
+          freeTrial: undefined,
+          serverOnly: false,
+          stackable: false,
+          prices: 'include-by-default',
+          includedItems: {},
+          isAddOnTo: false,
+        },
+      },
+      priceId: 'price-any',
+      quantity: 1,
+    })).rejects.toThrowError('Customer already has purchased this product; this product is not stackable');
+  });
+
+  it('allows when subscription for same product exists and product is stackable', async () => {
+    const tenancy = createMockTenancy({
+      items: {},
+      catalogs: {},
+      products: {
+        'product-sub-stackable': {
+          displayName: 'Stackable Product',
+          catalogId: undefined,
+          customerType: 'custom',
+          freeTrial: undefined,
+          serverOnly: false,
+          stackable: true,
+          prices: {},
+          includedItems: {},
+          isAddOnTo: false,
+        },
+      },
+    });
+    const prisma = createMockPrisma({
+      oneTimePurchase: { findMany: async () => [] },
+      subscription: {
+        findMany: async () => [{
+          productId: 'product-sub-stackable',
+          currentPeriodStart: new Date('2025-02-01T00:00:00.000Z'),
+          currentPeriodEnd: new Date('2025-03-01T00:00:00.000Z'),
+          quantity: 1,
+          status: 'active',
+        }],
+      },
+    } as any);
+
+    const res = await validatePurchaseSession({
+      prisma,
+      tenancy,
+      codeData: {
+        tenancyId: tenancy.id,
+        customerId: 'cust-1',
+        productId: 'product-sub-stackable',
+        product: {
+          displayName: 'Stackable Product',
+          catalogId: undefined,
+          customerType: 'custom',
+          freeTrial: undefined,
+          serverOnly: false,
+          stackable: true,
+          prices: 'include-by-default',
+          includedItems: {},
+          isAddOnTo: false,
+        },
+      },
+      priceId: 'price-any',
+      quantity: 2,
+    });
+
+    expect(res.catalogId).toBeUndefined();
+    expect(res.conflictingCatalogSubscriptions.length).toBe(0);
+  });
 });
 
 describe('combined sources - one-time purchases + manual changes + subscriptions', () => {
@@ -967,7 +1118,7 @@ describe('combined sources - one-time purchases + manual changes + subscriptions
 
 
 describe('getSubscriptions - defaults behavior', () => {
-  it('includes ungrouped include-by-default offers in subscriptions', async () => {
+  it('includes ungrouped include-by-default products in subscriptions', async () => {
     const tenancy = createMockTenancy({
       items: {},
       catalogs: {},
@@ -1012,7 +1163,7 @@ describe('getSubscriptions - defaults behavior', () => {
     expect(ids).toContain('freeUngrouped');
   });
 
-  it('throws error when multiple include-by-default offers exist in same group', async () => {
+  it('throws error when multiple include-by-default products exist in same catalog', async () => {
     const tenancy = createMockTenancy({
       items: {},
       catalogs: { g1: { displayName: 'G1' } },
@@ -1046,12 +1197,12 @@ describe('getSubscriptions - defaults behavior', () => {
       subscription: { findMany: async () => [] },
     } as any);
 
-    await getSubscriptions({
+    await expect(getSubscriptions({
       prisma,
       tenancy,
       customerType: 'custom',
       customerId: 'c-1',
-    });
+    })).rejects.toThrowError('Multiple include-by-default products configured in the same catalog');
   });
 });
 
diff --git a/apps/backend/src/lib/payments.tsx b/apps/backend/src/lib/payments.tsx
index 2a48af35f4..66b44f13cf 100644
--- a/apps/backend/src/lib/payments.tsx
+++ b/apps/backend/src/lib/payments.tsx
@@ -301,12 +301,19 @@ export async function getSubscriptions(options: {
   for (const catalogId of Object.keys(catalogs)) {
     if (catalogsWithDbSubscriptions.has(catalogId)) continue;
     const productsInCatalog = typedEntries(products).filter(([_, product]) => product.catalogId === catalogId);
-    const defaultCatalogProduct = productsInCatalog.find(([_, product]) => product.prices === "include-by-default");
-    if (defaultCatalogProduct) {
+    const defaultCatalogProducts = productsInCatalog.filter(([_, product]) => product.prices === "include-by-default");
+    if (defaultCatalogProducts.length > 1) {
+      throw new StackAssertionError(
+        "Multiple include-by-default products configured in the same catalog",
+        { catalogId, productIds: defaultCatalogProducts.map(([id]) => id) },
+      );
+    }
+    if (defaultCatalogProducts.length > 0) {
+      const product = defaultCatalogProducts[0];
       subscriptions.push({
         id: null,
-        productId: defaultCatalogProduct[0],
-        product: defaultCatalogProduct[1],
+        productId: product[0],
+        product: product[1],
         quantity: 1,
         currentPeriodStart: DEFAULT_PRODUCT_START_DATE,
         currentPeriodEnd: null,
@@ -426,18 +433,19 @@ export async function validatePurchaseSession(options: {
     },
   });
 
-  if (codeData.productId && existingOneTimePurchases.some((p) => p.productId === codeData.productId)) {
-    throw new StatusError(400, "Customer already has a one-time purchase for this product");
-  }
-
   const subscriptions = await getSubscriptions({
     prisma,
     tenancy,
     customerType: product.customerType,
     customerId: codeData.customerId,
   });
-  if (subscriptions.find((s) => s.productId === codeData.productId) && product.stackable !== true) {
-    throw new StatusError(400, "Customer already has a subscription for this product; this product is not stackable");
+
+  if (
+    codeData.productId &&
+    product.stackable !== true &&
+    [...subscriptions, ...existingOneTimePurchases].some((p) => p.productId === codeData.productId)
+  ) {
+    throw new StatusError(400, "Customer already has purchased this product; this product is not stackable");
   }
   const addOnProductIds = product.isAddOnTo ? typedKeys(product.isAddOnTo) : [];
   if (product.isAddOnTo && !subscriptions.some((s) => s.productId && addOnProductIds.includes(s.productId))) {
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/payments/before-offer-to-product-rename/outdated--purchase-session.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/payments/before-offer-to-product-rename/outdated--purchase-session.test.ts
index f5c6c0a5e6..8413043a84 100644
--- a/apps/e2e/tests/backend/endpoints/api/v1/payments/before-offer-to-product-rename/outdated--purchase-session.test.ts
+++ b/apps/e2e/tests/backend/endpoints/api/v1/payments/before-offer-to-product-rename/outdated--purchase-session.test.ts
@@ -800,7 +800,7 @@ it("should block one-time purchase for same product after prior one-time purchas
           displayName: "One Time Offer",
           customerType: "user",
           serverOnly: false,
-          stackable: true,
+          stackable: false,
           prices: { one: { USD: "500" } },
           includedItems: {},
         },
@@ -842,7 +842,7 @@ it("should block one-time purchase for same product after prior one-time purchas
     body: { full_code: code2, price_id: "one", quantity: 1 },
   });
   expect(res.status).toBe(400);
-  expect(String(res.body)).toContain("one-time purchase for this product");
+  expect(String(res.body)).toBe("Customer already has purchased this product; this product is not stackable");
 });
 
 it("should block one-time purchase in same group after prior one-time purchase in that group (test-mode persisted)", async ({ expect }) => {
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/payments/purchase-session.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/payments/purchase-session.test.ts
index e062c0a6d0..e3897cf8e0 100644
--- a/apps/e2e/tests/backend/endpoints/api/v1/payments/purchase-session.test.ts
+++ b/apps/e2e/tests/backend/endpoints/api/v1/payments/purchase-session.test.ts
@@ -800,7 +800,7 @@ it("should block one-time purchase for same product after prior one-time purchas
           displayName: "One Time Product",
           customerType: "user",
           serverOnly: false,
-          stackable: true,
+          stackable: false,
           prices: { one: { USD: "500" } },
           includedItems: {},
         },
@@ -842,7 +842,7 @@ it("should block one-time purchase for same product after prior one-time purchas
     body: { full_code: code2, price_id: "one", quantity: 1 },
   });
   expect(res.status).toBe(400);
-  expect(String(res.body)).toContain("one-time purchase for this product");
+  expect(String(res.body)).toBe("Customer already has purchased this product; this product is not stackable");
 });
 
 it("should block one-time purchase in same group after prior one-time purchase in that group (test-mode persisted)", async ({ expect }) => {