There are literally dozens of "make your own t-shirt" offerings online, most with a wider array of shirt colors and the ability to include your own graphics in addition to text, at a price comparable or cheaper than you are offering (cafepress.com, inktastic.com, etc). I've used these services myself and found them easy, not at all "cumbersome" as you state on helvetitee.com.
What value does your offering have in comparison to your competition?