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

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.

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