Lets say I address and stamp and envelope, but before I mail it I spill coffee on it. The envelope is usuable, but the stamp is still fine. Can I cut it off and attach it to another letter, or is that not allowed?
As long as the stamp is uncancelled, you can reuse it. I’m not sure if you’re allowed to cut it off, though – you may have to peel it off the soggy envelope. Also the PO may reject it if you put a piece of tape over it – glue it to the envelope.
I have done exactly that before, and taped the stamp to the envelope. It went through fine.
I think the PO sez that you can’t do that, but it’d be impossible to enforce.
it’s also quite possible to ‘uncancel’ a stamp and reuse it… not that i would ever condone such a unsavory and illegal practice to save 37 cents (soon to be 42) 
I’ve cut stamps from envelopes and glued them to new ones for years. Never had a single problem.
The reason that taping isn’t allowed is that the cancel mark will go on the tape, which could potentially be removed or washed and reused. I’ve had letters returned when I used tape over the stamp to hold it on.
You can also take a soiled, unused stamp (or one that, for instance, is affixed to an envelope you had to reopen before mailing) to the post office and get a replacement.
I tried it with tape once, and it was returned. I assumed it was because the infrared reader couldn’t scan it with tape over it.
Is it really going up to 42 cents?!
I think the real problem the USPS has with tape over the stamp is that the cancellation ink won’t “stick”; a quick wipe with water or solvent would make the stamp appear uncancelled and thus reusable.
I’ve always used glue and nobody ever had a problem.
Which, I believe, is the only correct way to do it. I had a guy at the PO peel a stamp off for me and put it on another envelope. If the stamp has been on for a while it won’t work but a freshly applied stamp can be peeled off.
You aren’t allowed to cut them off and glue them on a different envelope although I’ve done it and as has been said the prohibition is hard to enforce. There is virtually no chance it will be noticed at the sending end. They just collect the mail at the boxes and stuff them in bags without checking individtual envelopes. At big sorting and distribution centers they are too busy to worry about a single, piddling stamp. The only one who would have a chance to notice the violation would be the final delivering individual, and who would be so chicken-shit as to sqawk?
And probably no one will squawk either.