While it sounds like a good idea. Some of the mark up is to cover shipping, bad parts, wrong parts and other liabilities. You also will have the customer that is sure the part is the reason his lawnmower blew up. Sounds like a headache with little return. If you do do it 20% off is fair to both people involved. If they are a close friend then decide if you want to go deeper into your discount but keep that private.
Art