Is an unused stamp still valid for postage?

Answer 1

It depends on the country in which you live. In the US stamps never expire, so the answer is yes. ( There are a few old exceptions, but you are not likely to have any of these lying around. Also special delivery stamps which did not really pay for postage, but for a service that is no longer available, are not valid for anything anymore.)

Answer 2

Yes, it is still valid for the amount it was purchased for unless it was a Forever Stamp. You will have to add the appropriate stamps up to the current first class rate.