Customer Does Not Need Legs Because of Curb
If your customer does not need the back of the legs because they have a curb and they haven't bought yet, you can give them a credit and make a note in the design tool to not send "X" amount of legs.
The credit will be as follows, $4 (retail value of the leg) minus the discount that is reflected in their shopping cart.
If the customer already received the legs and they want to return them, they would have to pay for their own shipping and a 10% restocking fee.
In most cases, it doesn't make sense for them to return it because they would spend more money on shipping and the restocking fee then what the refund is going to be towards the end.