Quoted:
If your order is something they take off the shelf and put in a box, then yes, you should pay a restocking fee. But if it's something that goes in the "to be built" pipeline (e.g. an upper), and you cancel it before it's built, I don't see much justification for a restocking fee.
When you submit your CC info at purchase, a processing fee is taken off the top and the balance sent to us. When you cancel, another fee is charged to us based on a percentage of the amount refunded. The balance is to offset the costs of manually stopping orders, which involves taking an employee who was actively shipping orders and diverting them to finding the cancelled order and removing it from the shipping process. Sometimes this means just finding the order in a stack of paper, sometimes it means taking apart pallets of boxes, and stacking them back up, to search for the order.
The order must then be credited back to the customer, which takes manual action by another employee. So even if there is nothing to build, the order has already paid a percentage to a third party, and sets a process in motion that requires human intervention to stop.