I also want to mention that I see Paypal and Amazon accounting for this, but they are doing it in a way that it hurts you to use it on all transactions over $10-12. In my case this could happen if someone did bulk buying. How would you model this out in a test environment, so you kind of knew what to expect?
Use the micropayment apis for products under the micropayment structure ceiling. Then use other apis, non micropayments, for transactions over a certain limit. Amazon FPS switches for you.
I wasn't aware of the Amazon FPS. I am going to check this out and see how difficult it is to implement it. I guess the thing with using Amazon is whether people have more amazon accounts than paypal.
I would use Amazon and Paypal. While Amazon FPS scales to micro/regular payment for you, the Paypal link above mentions opening one micropayment and one regular paypal account and switching manually between. Both are allowed to get the best rate and it is worthwhile if you sell from .99 past $10+.