Hi Eric, welcome to the T-Shirt Forums!
Your best bet might be to go ahead and test the various free fulfillment services to see which works best for your needs.
It doesn't have to be as complicated as "auditing" the fulfillment house. They are established companies that show you ongoing sales stats.
Check out the offerings of
www.cafepress.com and
www.spreadshirt.com and see which one works best for you. There's no obligation to stay with either one once you've tried them out.
You'll find a lot of your questions may be already answered by previous discussions in the forum if you
check out the forum search.