WordPress.com blogs aren't intended for ecommerce. Even though the blog appears to be a nonprofit blog and the sales might be conceivably considered "donations," you might still be in violation of the ToS.
If you are actually making the shirts yourself, you can sell them through etsy -- which does not allow reselling -- and then link to your etsy store.
Basically, if you want an ecommerce site, you need to self-host with wordpress.org software or seek another solution on a different blogging platform.
Perhaps someone else has a more hopeful answer for you, but I think you'll need to think of a different venue for marketing your shirts.