I haven't dealt with the shirts yet since mine are sale as you go, but my policy on graphics work, such as blue prints, flyers, brochures, etc., 50% upfront the rest on pickup, only have 2 long time customers that I do invoice work with a 30, 60, or 90 day final payment but always 50% upfront for all incidentals.
These guys have done business with me since college, and we've worked together for various firms, small jobs they just pay up, but sometimes they have their money held at the other end, and they refer lots of work my way, so its not a deal just anyone gets. Don't know if shirts will fall into this area, the order would have to be ridiculous huge.