[Por_sha911]

This is way to complex. Make a firm rule, put up a sign (and put it on your paperwork), and stick to it 100% of the time. If the customer doesn't want to pay for your parts at your price then that's fine; you probably don't want them for a customer anyway. As long as they know up front how it is going to work then there should be no problem. I ask for a price quote before the work begins so I know what to expect when I come to pick it up. Like Sammy said, if the place slides in extra cost without telling me up front or, jacks me up on price more than on the estimate in a big way, then I won't be back.