I've ordered 5 times from GMPartsDirect (which is really Flowauto)
The parts take a while to come but they do come.
The larger problem is the customer service is limited.
They won't answer questions on parts, and the instant it ships they stop answering calls.
I ordered parts this last time and they sent me older parts that were deprecated.
I can't get an explanation from them on that.
It's not a great feeling installing an obsolete part.