Solve stamps from the envelope is easy
It happens to me from time to time that I have already stuck a stamp on an envelope and then see that I have committed myself to the address, or I want to pack something else, and then the postage is not enough and I have no matching Stamp.
So I easily remove the stamp from the envelope by moistening the inside of the envelope. This is easy and I can use the stamp with glue stick for another letter again.
When new stamps are glued together, I can easily detach them with a warm iron by swiping the iron over them.
Or, I try to buy the new, self-adhesive stamps, which do not stick together, but only in the postage box.