[glances around sheepishly]
Welcome to the Stingy Cad Club! I do this too. Yes, it’s perfectly legal as long as the stamp hasn’t been cancelled (and why would it be, unless the person who’s inviting you to the wedding is as stingy as you are and glued cancelled stamps to all her RSVP envelopes…)
It isn’t that it’s illegal to use glue or tape to affix a stamp to an envelope. It’s just that it usually makes the stamp too fat to fit properly in the cancellation machine. The machine will then spit it out for “hand stamping”, which leaves it at the tender mercies of some bad-tempered mail clerk who then gets to decide whether to hand-stamp it, or return it to sender (you) for re-postaging. Guess which one he’s going to choose.
Don’t use Scotch tape to stick it onto the envelope. The cancellation ink won’t “whomp” onto it correctly as it goes through the machine, and the machine will send it to the hand-stamping clerk.
Don’t use a rolled-up circle of inside-out tape, either. This makes the stamp too fat to fit into the cancellation machine, and it’s “hand stamping” time.
Soak it carefully off the RSVP envelope, let it dry thoroughly, then use a TINY bit of Elmer’s glue (just the merest film) to glue it on the envelope. You want to avoid big lumpy blobs of dried glue, which also screw up the cancellation machine, which means–guess what? That’s right, the “hand stamping” clerk.
Since Elmer’s glue tends to curl up the paper somewhat, you might want to weight it down with a book or something to make sure it dries absolutely flat. Why? So it doesn’t screw up the cancellation machine, of course…
Make sure the glue is thoroughly dry before you drop it in the mailbox, unless you want it to pick up all kinds of interesting passengers, in the form of other people’s mail.