When a customer brings a shop a part they want installed, it should be a universal policy not to do the work. Not to mention the loss in money for selling the part, but the legal liabilities.
Explaining why to the customer is difficult once they hear the words profit and selling. I recently had to explain to a good friend why a shop refused to install his “super duper” brake pads he bought from a German website. He thought it was a grand conspiracy.
Looking to back up my point, I ran across this explaination from Summit Transmission: