You want to supply parts? Fine. No warranty and labor goes up. Gotta keep the lights on, the insurance man happy and everyone else who has their hands out.
It's a profession- leave it to the professionals. If you think you know something, have a friendly conversation prior to agreeing to the work to make sure your opinion is heard and your wishes are followed or simply do it your self. Don't start cutting his throat on parts.
Too many people think they're getting hosed, but they really have no idea how much money is REALLY involved in a shop. Don't get me wrong- there are shops that will hose you! I used to work for one... But I bet more than half are just average Joes just trying to put some money away for their business to survive and for when they get old and gray. Why would you want to take away their livelihood?
When I had my roof done, it was by a customer. He doesn't whine about my prices... He either has the money, or he doesn't. He shot me a price, I deemed it fair and accepted. He came out and had a few questions- I told him that he was the pro, do what he felt necessary. When he was done, he handed me the bill (for which I was prepared) and I handed him money. Sight on spot. No whining.
The point is to relay any special wishes or info, make sure you get an accurate bid- then accept or decline. Let the mech do his/her job and make their money. You want to save money with interwebz parts then do it yourself and save even more.