I don't plan on participating, but I was curious if seals and hanko type seals and cylinder seals count towards this? They aren't strictly stamps, which were, apparently (maybe obviously), developed in order to determine if the postage had been paid for a letter, but they are sort of the same idea.

Carrying on Azelors thought, an envelope as a stamp (proof of purchase). You bring a letter to the post office, buy the envelope and it includes the cost of the postage for whatever is put inside, seal it up and drop it in the box.