Shirts are big fun.
Mine were designed by next door neighbor, a artist type guy and one of my customers prints them up for me.
More colors equals more money at setup and each printing.
In general XXL shirts are about .50 extra and each size larger adds up as well.
If you have camera ready art (generally made on computer, for easy color separation) it is cheaper at setup.
Generally you pay the setup fee once and then they keep the screens and can make more easily forever.
Think about who will wear them and where, likely at their shops and GTGs. If you are looking to drive traffic to your site, you want the simplest, easiest to remember address for that.
DDave