As a tradesman, I'm shocked to hear some machanics increase their labour rates based on the fact the customer brings there own parts. I know my mechanic doesn't, though being a perforance shop the hourly rate is soild to start with but so is the quality of the workmanship. As an electrician, if we increased our rate based on the fact the client supplied there own light fittings or switch boards they'd just laugh. The hourly rate set should cover the cost of running the business.
Op as many have said you'll never receive an itemised bill as all trade based businesses buy wholesale and sell retail. Get a quality mechanic, one that has transparency in it rates.