As an independent I am also able to base each case on its merits.
It isn't likely that I will recieve a car and give it back repaired using customer supplied parts. On the other hand we do many labor only or labor intensive jobs where a rate is established to cover the lack of parts mark-up. These are almost always where warrantee to the actual car is not implied. In other words when we spend 15 minutes to press in the 2 balljoints we have covered the spread and have very little warrantee concerns.
Bosch Master, ASE Master, L1
33 years MB technician