No Justification for Handling Fee

I ordered parts from Evetech (Desktop upgrade kit). I have installed the parts and my PC ran for a few days until it crashed. After testing i have determined that it is one of their parts that is faulty. I am now in the process of returning the parts, however, they state that they will take a 20% handling fee. They have no justification to take this handling fee. I have paid them for the parts, for their shipping and now they expect me to pay for the faulty parts to be shipped back to them. They aren't making a loss whatsoever because they will send the parts back to the supplier due to them being under warranty.

I refuse to pay this handling fee. Especially when the parts are brand new and faulty.