Amazon Simple PayTM is a set of payment-only products from Amazon Payments that allow your customers to use payment information from their Amazon.com account as a payment method on your website.
Amazon Simple Pay differs from Checkout by Amazon in that Checkout by Amazon provides additional capabilities to support real-time shipping and tax calculation, promotions, and order management-tasks including order cancellations, order reports and shipment tracking. If you don't need Amazon Payments' end-to-end checkout pipeline and order management capabilities, but still want to enable your customers to use their payment information already on file at Amazon, use Amazon Simple Pay.