You can check your Amazon credit balance quickly and easily using your computer, and reload it by heading to the "Gift cards" ...

I am constantly receiving Amazon gift cards as a thank you from various people, which I love because I know I will be able to redeem them quickly. But sometimes the cards don't have an amount printed ...

You can use a prepaid Visa gift card to shop on Amazon, but you'll have to work around Amazon's system to make it happen.