Has anyone used old stamps on their invitations? I'm using an assortment of stamps and some are 1960s and older (see pic) so their glue isn't always in the best shape. Plus I've dug through lots of dusty boxes with loose stamps to find these, so I'm not so sure I want to put my tongue on them. I did try licking the ones for the RSVP envelope and they seem to be sticking, but I did think back to that Seinfeld episode about licking envelopes...
What do you think I should use to make sure they stay put? Glue stick, tape gun, any other options? Suggestions greatly appreciated, TIA.

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