This is where things get challenging. If you imagine this system, if you try to walk through in your mind how this would work, it will probably occur to you to ask the question, "Okay, but... How does the system know that you shipped something?"
Yeah about that. I log into Patreon, and I basically make a little blog post that says "Look! I made the thing you pledged to pay me for!" with the associated ticky box for "Pay me for this" ticked, and click submit.
That's it.
Patreon and no way, shape, or form attempts to validate that I actually shipped the thing. That is between me and my patrons. And, personally, I'm smart enough not to use Patreon as my delivery mechanism, so delivery of what I am paid for happens through a completely other channel.