I'm pretty sure their service only prints on white shirts and the print is glorified iron-on.
I have used
iPrint in the past for shirts, but you have to buy at least 12 (maybe 24) and they're not exactly cheap. The quality of the print is very good though.
I know that at work, we used a place in NYC to do screen printing. It came out to only $7 per shirt and I don't think we bought very many. The shirt was a cheap brand and the print was only 1 color for our application, but I can find out who that was if you want.