It was my understanding that you could designate white as a color or a transparency but that you had to make note of it when you submitted the file for printing. At least that's how it worked with Spreadshirt.
I don't think any of the print on demand company's are great when it comes to quality. I can tell you a story about ordering 8 buttons from Spreadshirt and receiving fully half the order dented and damaged.
I can tell you another story about ordering shirts from Cafe Press that I still have today. For the post part they are all ruined because the image transfers flaked off (which made a real mess in my laundry) in the case of some of the shirts, peeled off in the case of the letters and generally could not stand the test of time.
By comparison the screen printed shirts I've ordered are still in excellent condition and though they show wear the graphics on them look brand new. And they cost me less per shirt than what I paid for the ones I bought at Cafe Press.
I think the best way to see which company to deal with is to design a shirt, open an account with them (free) and order your own shirt. If you do this for Cafe Press, Spreadshirt and Zazzle you will drop 45 bucks but you will have some idea of the best shirt quality. This is what I plan to do next month when I open my store.
