If the Mechanic in question owns his own shop he will most likely charge for the parts and they will be slightly more than could be bought by yourself loclly.

Some places the Mechanic who works in a shop owned by someone else also gets a percentage of the money from Parts as part of his pay.

Ounce your Car is in the Shop they try to look for more work that they can do quickly like changing Belts or Coolant Hoses.
If they have any sort of equipment like a Coolant Flushing Machine or for gas Engines an Injector Cleaning Machine they will try to talk you into having that done.

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84 300D, 82 Volvo 244Gl Diesel