When money has already been spent on something, it's prepaid, like a prepaid cell phone or credit card. You won't get a bill for using either, because they're both prepaid!
Anything that's prepaid has been paid for in advance. A prepaid phone includes a certain amount of talking and texting in its price, so you won't be invoiced by a phone company later. The same is true for a prepaid bank card; you paid for it before using the card. Prepaid was originally used to describe postage stamps, which signaled that a letter was prepaid by the sender, and the receiver didn't have to cover the cost.