If you've ever been on the shop owner side of the counter, you might have a different perspective.  I personally have not but this is how it was explained to me by a person who has:

Shops have two basic sources of revenue -- labor charges, and the markup on parts.

It's unlikely that you as an individual retail consumer can get parts "cheaper" than the shop can.  What you are seeing as cheaper is the shop markup.  100% is not uncommon.  Sounds like a lot but this covers some of the shop overhead and convenience to the customer.

So if the owner has his costs and prices calculated assuming a markup on parts, and you show up with parts in hand, he's now LOSING MONEY by doing the work for you.  And of course if he does this too much he's either going to have to charge more for labor or go out of business.  That's why many shops dislike or refuse to install parts you bring in.

Allan
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1991 Vanagon GL