naw, here's what probably happened:
Postal workers aren't allowed to accept tips. For some reason this dude was willing to risk disciplinary action for a nickel. But if he left the "here's a tip for your troubles" part on the envelope, he might get in trouble. So he tore that corner of the envelope off and dropped it in a "we damaged your mail" envelope, which does not require postage.
As to why he didn't take the 39 cents, I dunno, other than maybe he saw it was a joke and decided to play along
