So if I understand you correctly, if I have a $0 balance, and a $400 limit, and I prepay $100, I can the charge $500 in one go? Is that universal?
That's not what I meant to say, but you could probably do that. You should read the terms of your credit card to make sure.
For Amazon, you could purchase gift cards and add them to your account. You might be able to buy them locally. A gift card balance on your account reduces what you owe by CC.