Pretty much all the posts on that thread are reasons for the functionality: * Transaction fees that people have to pay on their credit card * Budgeting purpos

Pretty much all the posts on that thread are reasons for the functionality: * Transaction fees that people have to pay on their credit card * Budgeting purposes * Expense handling if you're buying the company services on a personal card (this happens a lot more than you'd expect) * Company shared accounts where the finance/purchasing department wants to do the old invoice / payment dance - finance department gets invoice for $5000, pays it in the background, developers get on with their work. * Applying a cap on your spending * Lack of credit cards in many parts of the world I'm sure there's dozens of reasons why you'd want to prepay, and it's clearly something Amazon want to support, judging by their staff's comments on the page.