We went with this Teespring service because they handle all of the printing and order fulfillment (e.g. shipping it to the folks who buy them). The caveat to that is that we can't just offer them for sale all the time, we have to do it in limited runs. But considering it keeps us working on Roll20 and not spending time printing and mailing t-shirts, I think it's for the best, at least for now.
