diff --git a/package.json b/package.json
index b53d665..adc15b8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@moneydevkit/api-contract",
-  "version": "0.1.16",
+  "version": "0.1.17",
   "description": "API Contract for moneydevkit",
   "main": "./dist/index.cjs",
   "module": "./dist/index.js",
diff --git a/src/contracts/checkout.ts b/src/contracts/checkout.ts
index 2349ed2..2c6b848 100644
--- a/src/contracts/checkout.ts
+++ b/src/contracts/checkout.ts
@@ -72,6 +72,14 @@ export const ConfirmCheckoutInputSchema = z.object({
 	 * Customer data provided at confirm time.
 	 */
 	customer: CustomerInputSchema.optional(),
+	/**
+	 * Product selection at confirm time.
+	 * - undefined or [] = keep current selection
+	 * - [{ productId }] = change selection to this product
+	 * - priceAmount required if selected price has amountType: CUSTOM
+	 *
+	 * Currently limited to single selection (max 1 item).
+	 */
 	products: z
 		.array(
 			z.object({
@@ -79,6 +87,7 @@ export const ConfirmCheckoutInputSchema = z.object({
 				priceAmount: z.number().optional(),
 			}),
 		)
+		.max(1)
 		.optional(),
 });
 
diff --git a/src/contracts/products.ts b/src/contracts/products.ts
index c293838..faba3ea 100644
--- a/src/contracts/products.ts
+++ b/src/contracts/products.ts
@@ -4,14 +4,14 @@ import { CurrencySchema } from "../schemas/currency";
 
 export const ProductPriceSchema = z.object({
 	id: z.string(),
-	amountType: z.enum(["FIXED", "CUSTOM", "FREE"]),
+	amountType: z.enum(["FIXED", "CUSTOM"]),
 	priceAmount: z.number().nullable(),
 	currency: CurrencySchema,
 });
 
 // Products have a prices array to allow future support of metered pricing
 // (e.g., base subscription + usage-based charges). Currently only one static price
-// (FIXED/CUSTOM/FREE) is supported.
+// (FIXED/CUSTOM) is supported.
 export const ProductSchema = z.object({
 	id: z.string(),
 	name: z.string(),
diff --git a/src/schemas/checkout.ts b/src/schemas/checkout.ts
index 1e90a21..1454e85 100644
--- a/src/schemas/checkout.ts
+++ b/src/schemas/checkout.ts
@@ -58,6 +58,29 @@ const BaseCheckoutSchema = z.object({
 	customer: CustomerOutputSchema.nullable(),
 	customerBillingAddress: z.record(z.any()).nullable(),
 	products: z.array(CheckoutProductSchema).nullable(),
+	/**
+	 * The selected product ID (from the products array).
+	 * For PRODUCTS checkouts, this is the product the customer has chosen.
+	 * null for AMOUNT/TOP_UP checkouts.
+	 */
+	productId: z.string().nullable(),
+	/**
+	 * The selected product price ID.
+	 * References a price from the selected product's prices array.
+	 * null for AMOUNT/TOP_UP checkouts.
+	 */
+	productPriceId: z.string().nullable(),
+	/**
+	 * User-provided amount for CUSTOM price products.
+	 * Only set when the selected price has amountType: CUSTOM.
+	 */
+	customAmount: z.number().nullable(),
+	/**
+	 * The selected product with full details (convenience field).
+	 * Same shape as items in the products array.
+	 * null if no product is selected.
+	 */
+	product: CheckoutProductSchema.nullable(),
 	providedAmount: z.number().nullable(),
 	totalAmount: z.number().nullable(),
 	discountAmount: z.number().nullable(),
diff --git a/src/schemas/product.ts b/src/schemas/product.ts
index 9e69f1f..c64c201 100644
--- a/src/schemas/product.ts
+++ b/src/schemas/product.ts
@@ -3,14 +3,14 @@ import { CurrencySchema } from "./currency";
 
 export const CheckoutProductPriceSchema = z.object({
 	id: z.string(),
-	amountType: z.enum(["FIXED", "CUSTOM", "FREE"]),
+	amountType: z.enum(["FIXED", "CUSTOM"]),
 	priceAmount: z.number().nullable(),
 	currency: CurrencySchema,
 });
 
 // Checkout products have a prices array to allow future support of metered pricing
 // (e.g., base subscription + usage-based charges). Currently only one static price
-// (FIXED/CUSTOM/FREE) is supported.
+// (FIXED/CUSTOM) is supported.
 export const CheckoutProductSchema = z.object({
 	id: z.string(),
 	name: z.string(),
diff --git a/tests/contracts/checkout.test.ts b/tests/contracts/checkout.test.ts
index 4336b23..997b968 100644
--- a/tests/contracts/checkout.test.ts
+++ b/tests/contracts/checkout.test.ts
@@ -205,9 +205,6 @@ describe('Checkout Contracts', () => {
             productId: 'product_1',
             priceAmount: 500,
           },
-          {
-            productId: 'product_2',
-          },
         ],
       };
 
@@ -230,8 +227,7 @@ describe('Checkout Contracts', () => {
       const input = {
         checkoutId: 'checkout_123',
         products: [
-          { productId: 'product_1' },
-          { productId: 'product_2', priceAmount: 1000 },
+          { productId: 'product_1', priceAmount: 1000 },
         ],
       };
 
@@ -239,6 +235,19 @@ describe('Checkout Contracts', () => {
       expect(result.success).toBe(true);
     });
 
+    test('should reject products array with more than 1 item', () => {
+      const input = {
+        checkoutId: 'checkout_123',
+        products: [
+          { productId: 'product_1' },
+          { productId: 'product_2' },
+        ],
+      };
+
+      const result = ConfirmCheckoutInputSchema.safeParse(input);
+      expect(result.success).toBe(false);
+    });
+
     test('should accept custom fields from confirm input (form fields)', () => {
       // Custom fields are accepted at confirm time - they come from the form
       const input = {
diff --git a/tests/index.test.ts b/tests/index.test.ts
index a17ef18..d7beb3c 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -60,6 +60,10 @@ describe('API Contract Index', () => {
             currency: 'USD',
           }],
         }],
+        productId: null,
+        productPriceId: null,
+        customAmount: null,
+        product: null,
         providedAmount: null,
         totalAmount: null,
         discountAmount: null,
diff --git a/tests/schemas/checkout.test.ts b/tests/schemas/checkout.test.ts
index 66ee657..a90d4a3 100644
--- a/tests/schemas/checkout.test.ts
+++ b/tests/schemas/checkout.test.ts
@@ -23,6 +23,10 @@ const baseCheckoutData = {
   customer: null,
   customerBillingAddress: null,
   products: null,
+  productId: null,
+  productPriceId: null,
+  customAmount: null,
+  product: null,
   providedAmount: null,
   totalAmount: null,
   discountAmount: null,
diff --git a/tests/schemas/product.test.ts b/tests/schemas/product.test.ts
index dd90f44..ebe61b5 100644
--- a/tests/schemas/product.test.ts
+++ b/tests/schemas/product.test.ts
@@ -43,7 +43,7 @@ describe("Product Schemas", () => {
 			expect(result.success).toBe(true);
 		});
 
-		test("should validate price with FREE amount type", () => {
+		test("should reject FREE amount type (not supported)", () => {
 			const freePrice = {
 				...baseProductPriceData,
 				amountType: "FREE" as const,
@@ -51,10 +51,10 @@ describe("Product Schemas", () => {
 			};
 
 			const result = CheckoutProductPriceSchema.safeParse(freePrice);
-			expect(result.success).toBe(true);
+			expect(result.success).toBe(false);
 		});
 
-		test("should reject METERED amount type", () => {
+		test("should reject METERED amount type (not supported)", () => {
 			const meteredPrice = {
 				...baseProductPriceData,
 				amountType: "METERED" as const,
@@ -275,18 +275,6 @@ describe("Product Schemas", () => {
 						},
 					],
 				},
-				{
-					...baseProductData,
-					id: "product_free",
-					prices: [
-						{
-							...baseProductPriceData,
-							id: "price_free",
-							amountType: "FREE" as const,
-							priceAmount: 0,
-						},
-					],
-				},
 			];
 
 			products.forEach((product) => {