This is completely normal and commonplace. Garages buy wholesale and then charge "list" for the price. I worked at napa for a few years and sold to just about every garage within a 30 mile radius like that. That's why a lot of places do not like you bringing your own parts (not to mention they have no control over quality etc).

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth