Does pre-paid first class postage expire?

Answer 1

Not in the UK or the United States. You can use them even when the price has gone up after you bought them, as long as the face value adds up to the correct rate.

Answer 2

{| |- | No, they do not expire. The US hasn't devalued postage since the Civil War. As long as you have the proper total value on the envelope the stamps can be added in. |}