I think that cheap shirts is something that people would definitely want. Though of course when I say cheap I mean in price not in quality.
Having a Mint store on this website that sell shirts would probably help in the costs that it takes to create and maintain Mint. Should probably talk to clem about incorporating shirts into a store. If you are able to make the shirts yourself and not outsource the work to someone else then it would be probably a good idea to have you make the shirts so that Mint and yourself gets the money.