From their site: 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.
Amazon has cemented itself as a top tier option for charging users thanks to its Amazon Flexible Payments Service API. The crowd-funding site Kickstarter has run hundreds of millions through the API, so it should also get some credit. Although, at Amazon, Kickstarter may get blame instead, having inspired many other sites, which eventually caused Amazon to turn away sites with similar concepts.