This has probably been stated several times by now, but the amazon documentation is absolutely horrible. Braintree, Stripe, and Paypal all have fantastic tech and reliable services in addition to fantastic documentation. I don't have an opinion for any other payment-processing vendors.

Now for the war story: Business was focused on deploying amazon-payments because of the cheap fees. In order to change their minds, I had them call into tech support. After 3 hours and 5 hangups, business opted for another vendor. The implementation was effortless and only took me a few hours.