If ever there were a character defined by his name, Stamp Paid's the guy. In fact, he straight out tells us the story behind his name. That means you can stop racking your brain for the "larger significance." Actually, scratch that. This is still a Toni Morrison novel, after all.
Anyway, he's the story: Stamp Paid is the name he chooses for himself after he gives his wife up to their master's son. His rationale goes like this:
Born Joshua, he renamed himself when he handed over his wife to his master's son. Handed her over in the sense that he did not kill anybody, thereby himself, because his wife demanded he stay alive. Otherwise, she reasoned, where and to whom could she return when the boy was through? With that gift, he decided that he didn't owe anybody anything. Whatever his obligations were, that act paid them off. (19.184-85)
In other words, Stamp Paid views his wife as a form of payment for his services or "obligations." It's unclear what happens afterward, whether he's eventually freed for giving up his wife or if he escapes on his own. What is clear, according to him, is that his name signals a new way of living, a "debtlessness" (19.185) that he decides to extend to anyone who wants to live debtlessly, too.
Translation? He helps runaway slaves escape a life where they can't own themselves, a.k.a. that whole ferrying across the Ohio River job. He delivers slaves to freedom.
But isn't Stamp kind of a chauvinist jerk-face for thinking of his wife as payment? That's a toughie.
See, sadly, Stamp and his wife don't really have a choice about where his wife ends up; they're slaves. Thinking about his wife as a "gift" that he chooses to give away is a little like pretending he has some right to his wife and to himself. It almost puts him on equal footing with his master. Of course, we can't really feel good at all about that reasoning.
Just to add to that uneasiness about Stamp, he tells Paul D a slightly different version of his story. In this version, we find out that his wife's name is Vashti (a Biblical character—in other words, "Alert! Deeper meaning!") and that he never really even had a chance to be with her because the master's son took her for his own.
Oh, and we also get this teeny, itsy, bitsy piece of information from Stamp: Vashti did finally return to Stamp, but when she did, Stamp only felt like snapping her neck (25.233). Yep. Stamp's got some serious anger issues. He tells Paul D that he didn't really snap her neck, but… um, yeah.
The point is, Stamp's a pretty complicated minor character.
He's generous and kind; you probably wouldn't want anyone else around your baby. (Remember how he's willing to suffer thorns and insect stings to pick berries for baby Denver? That's devotion for you.)
But like everyone in this book, he's got a dark side that he suppresses in order to be the kind of guy everyone welcomes into the community. How he finds it within himself to convert into that helpful "good guy" is definitely something that's worth going hmmm about.