I also reuse uncancelled stamps (I guess everyone does), but you are not supposed to. Per online Post Office FAQ:
" * Is it possible to reuse stamps?

A used stamp cannot ever be reused. If the stamp was already used to send mail through the USPS, it cannot be used to mail a different mailpiece even if the stamp was not canceled."

I have two amusing stories about postal clerks. Both happened recently:

(1) I was mailing an inexpensive chip to England, and it was worth it to tape it inside a letter, and mail it First Class LETTER rate,as it was no more than 1/4" think. At the last moment I decided to write "fragile" on the envelope. The clerk said that would make it First Class PARCEL rate! So I crossed out "fragile" and she accepted it as letter rate! She seemed pretty smart; maybe she was right, but I have dropped such "fragile" letters in the mail before and got away with it.

(2) We have a busy post office in Brooklyn, long lines at times, so they have a customer support person to advise people on line. This woman said she worked with postal standards before. I discussed with her the issue of putting the address NOT parallel to the longest side of a mailpiece because it didn't fit. She commented that doing so would make a first class letter "unmachinable", and thus there would be a surcharge ($0.20). She is right about that. .... ..... But she also said that it would apply to a PARCEL too. I am pretty sure she is wrong about that. I have been misaligning the address for years on parcels with no problems.

Robt.
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Timothy Dalton, as James Bond, and Carey Lowell in "Licence to Kill" (1989).
http://www.antiquegamblingchips.com/stillsHollywoodStarsGambl.htm