Prepaid means paid for in advance, before you use it. A prepaid envelope you drop in the postbox without buying a stamp.
The word attaches itself to whatever has been settled up front: a prepaid phone, a prepaid card you load with money, prepaid postage, even a prepaid funeral. Its appeal is that no bill arrives afterwards — you spend what is on it and then it stops, which is why prepaid cards are sold to people who would rather not have credit. In accounting, a prepaid expense is one paid for in a period earlier than the one it belongs to.
1adjectiveprepaidPaid for before being used or received.
“Send it back in the prepaid envelope.”