When I have a letter to send, I take a couple of strips of clear tape, and attach the stamps at the bottom and the top to my new envelope. I am careful to leave most of the stamp uncovered by tape, so that it's easy for the postal service to postmark the stamps (thereby invalidating them).
I've been doing this for many, many years without any problem. I've never had an envelope returned when doing this - until today!
Above you can see the envelope that was returned to me (FYI - before taking this photo, I had started peeling off the tape from the stamps, but then stopped because I thought this would make an interesting blog post. There was tape over the top of the stamps, and the stamps had not been abraded, when I originally mailed the letter). It is marked "RETURN FOR POSTAGE" and "Re-used Stamps". I figured someone at the post office was unfamiliar with the idea that it's okay to reuse a stamp as long as it has not been postmarked. So I went to the post office to clear things up. I thought that perhaps I had overdone it with the tape this time (a bit too much tape covering the stamps), and I was hoping someone would just cancel the postage for me at the counter.
Well, I was told that taping over stamps is "destroying government property," and it is absolutely not allowed. I explained that the stamps were not "reused" - they had not been used to send anything through the mail and were free of postmarks. Nope! Not allowed!
Yet for many, many years I've been getting away with this. I asked if it would be okay to glue the stamps on. They would neither confirm nor deny whether this was permitted; but at least they didn't say that this was "destroying government property."
I've been searching the internet to determine where the ruling is that you cannot tape stamps to an envelope. Rumors abound, but I have found no definitive answer. I still suspect that the real problem, in this case, was that the stamps were covered with too much tape. If I'd been more careful and exposed more of the stamp surface, this problem would never have occurred. The USPS apparently uses machines which scan the surface of the stamp to check whether it has been cancelled: http://pe.usps.com/text/csr/PS-281.htm:
Most stamps are coated with a substance that may be detected by cancellation machines which verify that postage is affixed. These machines reject letters on which a stamp cannot be detected.So yeah, I suspect that you can just get away with taping over stamps, provided you leave most of the stamp surface uncovered. But from now on, I'll use glue.