I posted this many years ago but it is still relevant:

A client was being a jerk and even though I saved him a lot under an audit, he still rejected my invoice. He also returned the "Sign Here" tabs and said I could re-use them. (True, but still an AH move.)

For the next invoice, I calculated my cost of the tabs, $X per 1,000 tabs, and included a credit for the returned seven tabs. Probably about 1.1 cents each. His response? "That wasn't necessary."

By the way, when he stiffed me for the fee, I added $$ to his next year's fee, and the next year's fee, ad infinitum. I made that "contested" fee back many times over.