It’s completely free, and they seem to keep it well updated, and it has plenty of features. If you’re JUST selling teeshirts, and there’s not a lot of variability to what you’re selling, then many of Magento’s features might be wasted on you. I’m not sure how much computer/web development experience you have, but Magento will have a learning curve. There’s a decent amount of documentation, though, and the setup process is pretty straight forward. Hardest parts will be setting up shipping/taxes/payment options, etc. and integrating with your current site.

It is a full-feature cart program, so it’s a little on the bloatware side, in my opinion, so you might also consider building your own (for simplicity and transparency) and/or also selling through seller channels like eBay, Amazon, etc. Of course Yahoo also does stores.