As I said, there are exceptions for every rule. Sometimes it's a tax issue, sometimes it's special order materials, sometimes it's equipment rental.
The main thing is that you pay your expenses. Some customers will try to "cherry pick." Others will try to dissect the billing to try to get a better deal. You've got to keep your eye on the ball.
You're there to make something work; you're not there to debate billing philosophy. You have enough non-billable time to cover, without adding hours of debate with the customer to the total. Never think you can win an argument with a customer; even when you 'win,' you lose. A bad experience means you don't get repeat business. It's best to not provide any opportunity for this to happen.
