In general, ftrotter is true. However, I wanted to include some features that make FPS different from Simple Pay. FPS allows you to set up “merchant” accounts for users, which allows you to manage both buyers and sellers (such as merchants). So, if you launched a website that sold handmade toys, you could allow merchants from all over the country to sell their toys on their website and allow customers to buy these toys without having to deal with money or a product. Although money never transfers your hands / account, you can set FPS to a percentage and / or a fixed amount of each transaction. This amount simply gets into your account every time someone makes a purchase. This is unique to FPS, although I believe PayPal and others also have a similar service.
If you do not need to have several sellers on your site, I would avoid FPS. Although they provide sufficient documentation for the initial setup, there is no troubleshooting documentation when the transaction fails and they fail. Also, if you don’t have enough money in your Amazon account, transactions will fail. But Amazon doesn't let you know what that value is, and it changes all the time. Literally, a transaction will start to fail because we are taking too much.
Good luck.