I can't claim to know much about how these things work, but I'd imagine that this dude is selling the shirts for more than it costs to make them for a bit of profit right? what if he gets to advertise on the site in exchange for giving a percentage of any profits he gets through sales on here back to you carbon?
Obviously I don't know how much he makes on each shirt so it may not really be enough to be worth getting a percentage of, but wouldn't it at least be worth talking to this guy to see if the two of you can work something out?
it might not turn out that you get enough to be happy with so you don't go ahead with it, but wouldn't it be worth having a talk with the actual guy to see what could be worked out, if it can go towards maintaining site costs and whatnot?
just a suggestion anyhoo :)