Bests & Worsts Reviews from Amazon

OK... *Deep breath*. I'm simply trying to make a payment. Why on earth would you require a CASE SENSITIVE USERNAME that's different from my Amazon username and a separate password in order to make a payment. Do you have problems with people making random payments to other people's credit cards?!?? For account information I can kind of understand it, to make a PAYMENT though? Of course they're willing to send you this username if you supply the number off the physical card that I of course would keep on me what with it being accepted everywhere! Oh wait, that's right, it only works on Amazon online! So to recap, you're willing to allow me to SPEND money with amazon at the mere flick of a finger, but if I want to pay that money back I have to jump through hoops? Why isn't it as easy to PAY as it is to spend? Why are the accounts not fully integrated? My login to Amazon seems to be proof enough that I am who I say I am to buy something. Why is that not enough to allow me to pay for it?