Does anyone know which of the three payment providers (Paypal, Google Checkout, Amazon) end up charging the least, so as to maximize my amount that goes to the devs/charities/etc?
I believe that Amazon's and Paypal's rates are exactly the same, but Amazon picks the cheapest rate automatically depending on if you pay more or less than $10 or so (it automatically considers it a micropayment and charges the appropriate rate), so I'd go with that.
PayPal charges a standard rate that you pick (micropayment or not), and if the guys thought most payments would be under $10, they might have optimised for that, so your purchase would be more expensive with PayPal.
PayPal charges a standard rate that you pick (micropayment or not), and if the guys thought most payments would be under $10, they might have optimised for that, so your purchase would be more expensive with PayPal.
TL;DR: Just pick Amazon.