You shoot yourself in the foot if you insist on supplying your own parts. First, you limit yourself to the lower tier mechanics who will take on any kind of job because they need the work. Second, you have no recourse on the outcome of the job, if/when it fails. If it doesn't fit, you are out the cost, so far, for the mechanic's labor, unless you chisel him out of his labor and his materials margin.
Just bad policy.
I have no problem with a shop obtaining the parts but I do have a big problem with them charging me more $$ for the parts than what I can buy them over the counter for...that's just outright gouging as I feel certain that shops get a discount on purchases so why gouge the client?
