I have to give Amazon credit – they’ve put together most of the things you need to assemble to make life easy for developers. They have a compute environment, bandwidth, affiliate products, an existing customer base, and a way to accept money. What more could a developer ask for from a platform provider?
True cost of processing for developers – Most payment processing businesses are based on some kind of wedge model. Charge the merchant x when it costs you less than x to actually process the transaction. Providing true cost of processing (or a closer approximation than PayPal is offering) is a pretty bold move. Importantly, it ought to be sustainable over the medium-to-long term.
Support for existing Amazon accounts – The other tough thing about building a payment system from scratch is getting an installed base of user accounts. Amazon has tens of millions of users who have transaction histories that can prove their (un)trustworthiness over time and an affinity for and trust relationship with Amazon. That’s a big boon to any developer looking to deploy the service.