Personally, I would charge more for customized shirts then shirts
. I would prefer shipping included in the price and I think it would make it easier for you to include it as well, ups or fedex. If you don't have a merchant account with them they're easy to set up and the shipping isn't that expensive. Because you're selling goods you may have to charge sales tax and such, if you have any questions for the accounting side of this pm me. I'll help you set up a basic spreadsheet so you can track costs and whatnot, and let you know which government forms need to be filled out if you want to go the whole "legal" route of selling merchandise.
edit: fixed a typo