Gateway site: https://payments.amazon.com/sdui/sdui/index.htm
Overview of Amazon Simple Pay
Amazon Simple Pay is a set of basic payment-only products. Using online Amazon Simple Pay forms you can create fully-functional payment buttons, which you then add to your web page. These buttons allow your customers to use their Amazon.com accounts as a payment method for purchasing products and services from you. When customers click one of your Amazon Simple Pay buttons, they begin making a secure payment.