I cannot recommend http://progressofdesign.com/ enough. I've ordered from them 3 times and couldn't be happier. Cheaper prices than most other places I've used in the past and great quality. I wanted to print a larger than average design and they accommodated me for only a small bump in price when other companies said they couldn't even do it.
I bookmarked Threadbird - http://www.threadbird.com/ recently, from the folks of Storenvy I think, but your pricing is much more competitive! Threadbird got a bit more printing options and materials.
we use vistaprint, primarily because the UI makes it easier to design shirts. You should test out the quality of shirts on a short run first.
If you keep an item in your cart, they will send you a coupon. If you buy a small amount, they will send you a coupon that you can use on a bigger order.
I've wanted to rekindle "Startup Schwag" as I didn't get to get on it for very long prior to its close down.
I've used Queensboro.com. Tons of options, and solid pricing. Would definitely use them again.