Been using Bay Photo for a few years. They're great.
Fast processing, fast shipping, reliable, and good quality products. Plus they offer large and custom sizes.
They also offer a drop shipping option that doesn't include any mention of Bay or pricing.
Another thing I like is no minimum order. Want to drop ship a 4x6? No problem. Some of the other labs have $20 or more minimum orders. I sell a lot big prints but also a lot of 8x12s.
Support has been great. Anything damaged during shipping they've immediately replaced and handled the issue with the carrier behind the scenes.
I've tried a few other labs with mixed results. One lab "overlooked" my order and after probably a week I called and they gave me a "so what oh well" attitude. Never used them again.