How to Get a discount on parts from the DEALER

All repair shops get a 15- 20% discount on parts from the dealer. This worked twice for me. I called up Pepe Mercedes in Westchester, needed some parts for ML55. I told them that im calling from Gruda Auto (which is 3 blocks from my job)and need such and such parts and would be in to pick them up in an hour. When I arrived at the dealer I told them that im making a pickup for Gruda. They gave me the parts and on the reciept there was a 20% discount applied![]