Rebecca,
I use a professional lab, Millers. I only place an order when I get an order from a client. They only charge $6 to drop ship to the customer. They never send the receipt with drop shipments so your mark up is not known to the customer. I like this route, because I don't have to have the prints on hand and store them. I also don't have to order them, wait until they get to me, just to turn around and ship it out. Plus some prints are really large and there is no way I could ship it as cheaply as Millers does. Everything is $6 and it is FedEx overnight. I have found this to be the best way of going about it. I hope this helps you.