Amazon Pay optimizes the buyer experience to reduce checkout time and improve conversion. Note that buyer-facing features do not require additional development effort to enable and will not change your integration.
Buyer default preferences
Amazon Pay skips the payment instrument and shipping address selection page for one-time checkout using PayAndShip
productType unless the buyer’s default payment instrument or shipping address can’t be used to complete the transaction. You must make sure you display the buyer’s default preferences and enable updates in case the buyer wants to select a different payment instrument or shipping address.