From their site :
Amazon Flexible Payments Service (Amazon FPS) is the first payments service designed from the ground up specifically for developers. The set of web services APIs allows the movement of money between any two entities, humans or computers. It is built on top of Amazon's reliable and scalable payment infrastructure.
Amazon FPS offers developers unmatched flexibility in how they can structure payment instructions, including standing instructions that can remain in place for multiple transactions. These instructions impose conditions and constraints on money movements and can be set by both senders and receivers of funds. For example, a sender might set a spending limit per week for a particular named recipient. Only that named recipient would be able to withdraw funds and only up to an amount per week equal to the spending limit. A piece of FPS functionality called the GateKeeper automatically enforces the constraints you set with payment instructions. When the sender or receiver is a computer system, payment instructions are set programmatically using APIs. FPS also provides a simple set of user interfaces that humans can use. From the users' point of view, they simply see terms of service and a request to accept those terms.
You can use the extensive feature set of Amazon FPS to conduct a wide variety of transactions under virtually any set of constraints. Key features include:
Send and receive money using credit card, bank account or Amazon Payments balance transfer as payment methods.
Create "Payment Instructions" to define conditions and constraints desired for a given transaction, and programmatically obtain payment authorizations or "tokens" that represent these Payment Instructions from customers.
Execute one-time, multiple, or recurring payments on behalf of customers.
Aggregate micro-transactions into a single larger transaction using Prepaid and Postpaid capabilities.
Build payment applications where you are neither the sender nor the recipient of funds. You can build marketplace applications that enable the movement of money between two third parties.
View account balances, transaction histories, and transaction details on the Amazon Payments web site.
Utilize the Amazon FPS sandbox to build and test applications without using real money or incurring any transaction charges.