I tried out Teespring- they sort of blew resolution wise. They might have a DP option for high res prints like I wanted to do, but it wasn't clear that I needed to select it if they did. In the preview pane, I could see the writing on the back of the shirt.. and it is totally illegible in the shirt that I ended up printing.

Sort of pisses me off, although CustomInk says for regular screen printing, you need letters to be .2" high for them to be legible. About twice what the lettering on my shirt is. Maybe I missed the fine print over at TeeSpring- but I doubt it.