As noted in another thread, you never prepay anything specific. You just put whatever money you want into your account as a credit balance, and that is used against your total bill at the end of the cruise.
In your own mind you can think of it as prepaying something in particular (people often talk about "prepaying tips" for example), but that's not how it works. If you want to give Disney some of your money ahead of time for some reason, feel free, and they'll use it to pay off whatever charges you accrue (tips, wifi, any of 50+ different categories you can spend money on).