Well, as a professional tech, I'll chime in.
Our policy is simple, you want to bring your parts, so be it. There is a add'l 10% added to the labor, there is no warrenty implied or otherwise on the repair. This includes the diagnosis etc.. However, this changes nothing in the manner of which your problem is handled, diagnosed and repaired. It simply allows me to recover some of the lost revenue from parts markup. Now, on the off chance I simply make a mistake while installing your part and do it incorrectly, that is warrentied.
I dont really like installing BYOP parts as there are issues about quality, accuracy of the diagnosis and overall completness of the repair that can and do rear their ugly heads. Personally I dont think the relationship between me and the client is worth posibly damaging over something so simple as a failed part.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.