Is there a difference on your end between a pre-order and a normal sale? Does one have benefits for you that the other doesn't? I like to buy stuff at the shows, but I'll pre-order if it helps more.
Preorders allow you to reserve the size you want. At shows, there's a chance I'll be out of some or all sizes depending on how well a shirt does and where in the tour the specific show is. It takes less work for me to sell a shirt at a show since I don't have to pack it and drive it to the post office and wait in line to get it to you.