Looks like we're both looking for the same thing, although in my case it's not initially for t-shirts. I checked out the site you referenced, and it's close to if not even the same interface as one I was trying to emulate, which is customink.com. They also have a user demo which is well designed and helpful.
I did find a few potential flaws in the (customink) design, which might be corrected when an order is placed, I didn't get to that point. In their case it is possible to increase the size of the text beyond the boundaries of the box inset, even beyond the substrate.