Uh, here since forever post office put rubber stamp mark on stamps to prevent that, I'm surprised there is country that doesn't

IIRC, there were two kinds of cancellation ink - black and red. The black just dissolved in isopropyl alcohol, but red was permanent. Probably the result of some cost / benefit analysis.

Yeah exactly. How do you go around the ink on the stamps? Ours ( French ) don’t go away with alcool. At least it was not the case in the 90´s.

Now you can just buy bootleg stamps on Amazon. Search "Forever Stamps". Amazon catches them frequently but then they just liquidate the stamps. My local Amazon returns store has them upfront 100 for $10.00 and they have tons of them.

We used to just cover the entire envelope in Sellotape and put an address label over the top so we could just wipe off the postage mark - a dated ink stamp over the postage stamp (Royal Mail employees circa 1990 didn’t get paid enough to care) and use it again.

If you regularly swapped disks with someone you’d just mail the envelope along with the disk