Product Advertising API
Developer Guide (API Version 2013-08-01)

Implement the Proceed to Checkout Button

Amazon hosts the web pages in the Order Pipeline. Your application plays no role in purchasing the items or order fulfillment. However, your application must use the PurchaseURL returned by Product Advertising API cart operations to add the shopping cart items to the Order Pipeline. The PurchaseURL is often an HTML form and button labeled, for example, Proceed to Checkout.