There are no rules, but there is convention, ethics, and respect. Due to convention your question will threaten the ethics and respect indirectly of a great body of respected technicians.
Since I obviously don't want to get in an economic discussion with a person whose obvious intent is to reduce my pay, I always leave the issue to warrantee. Ha, is all I say to the concept of warrantee of labor only. Brothers have shot brothers or such discussion. There is absolutely one way to resolve that warrantee issue and that is to warrantee the job, no questions asked.
And yes I have installed more owner supplied parts than most DIYers will ever need in a whole lifetime. We are an authorized installer for Tire Rack and are eager to avoid High Speed Tire warrantee problems by letting people buy their own tires and we just charge for mounting and balance. This is the type of exception that equals the wine example. To a real business asking such a proposal is tantamont to the syeak question at Ryan's. Of course if you have dropped a couple grand here last year I'll probably do anything you ask, no questions asked.
As to a bunch of techs available for such work, I doubt it. I am only including MB techs in the gene pool. There may be significant number of tire store techs and a wide range of shade trees
Bosch Master, ASE Master, L1
33 years MB technician