You might consider adding an option to display a QR code on the shirt instead of a URL. It would be really cool if you could also provide the ability to embed a logo/icon into the QR code (a la http://hackaday.com/2011/08/11/how-to-put-your-logo-in-a-qr-...). Would be good for companies.

I was a little disappointed that it isn't possible to order shirturl.com/1337 (minimum 5 characters).

The 'Home' link in the footer is a 404 (https://shirturl.com/home).

I think you should add a privacy policy.

That said, good luck!