I almost never use a customer supplied part unless I know what they have (example: I installed the customer supplies AC Delco plugs since this is what I would have used in the vehicle anyway). One of the things that I am learning the hard way is if you install a customer supplied part, you lose all the way around. If they bring you the wrong part, you are waiting on them to provide the correct part. You lose money on the parts markup which means you lose on the payment of your overhead. I don't give the customer the option. I tell them I would rather get the part then I know its correct, especially since most people don't have any idea things like engine size, year, trim level, and ect.