I spent the time and effort to find exactly the parts I wanted on my car at the price I wanted, a service that the shop isn't going to offer me.

As a repeat customer I would have been insulted if they upcharged me on the labour just because I brought in parts they could not offer me anyway.

And I'm guessing the people who bring their own parts are typically knowledgeable repeat customers who likely bring in enough word-of-mouth business to the shop to offset whatever profit may be made on the parts.

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