I don't think T-shirts are a good idea: different colors, sizes to be kept in stock. And they cost a bundle. I know that SKYMTL was joking when he wrote about the vinyl stickers, but that would be a great idea: one size-fits-all, cheap, and easy to ship.
With Spreadshirt the nice thing is Gav and can setup and the design and then people can order as needed. No overhead. People order and pay for the T-shirts through spread shirt. Gav can even set a specific price. They accept Credit Cards and PayPal.