The letter with the torn labels sometimes, but never fall off an envelope.
I was told by the Greek postal clerk that tape is "forbidden" across postage stamps.
Of course I then said that the company that makes the postage stamps should put GLUE on
the side so they will stick! Greek Post does NOT have self-stick stamps( ...one rare case was the
"Thessaloniki" punk stamp, which is hard to find). Self-stick postage stamps are the only answer for
slippery clear plastic , or slippery magazine page envelopes. Or, I will use all-purpose UHU glue,
and not even attempt to "lick 'n stick" :-)
Have you tried double sided adhesive tape behind the stamp?
Same reply from La Poste recently to another question: stamps cannot be altered in any way.
Clever idea - thanks!