I'd love to see a user journey map of how stamps would be used. Like, I sign up for a website and enter my email address and the open my mail client to generate a couple tokens, then I copy some blob of text into the sign up page (so they don't need to buy tokens). Then after a year of using the service, they run low on stamps and notify me (maybe over email) that they need more stamps? That sounds like a bigger hassle than spam (I don't get much).

Or am I misunderstanding how stamps would be used?

kick the can to Authentication and tie email issues to the rest of our lives... notx.