We look our bills over and sometimes find a mistake. But I can not remember ever finding a mistake that went in our favor.

Just put tires on a tractor, price quoted included mounting. When the bill came, there were charges for mounting, plus a charge for taking duals off to get to the inside tires to change them. Called the store, manager said he'd check into it. Called back and left a message that he'd take off the money for mounting, I had to call back a second time to remind him that the quote included removing/reinstalling the duals.

Yes, it does pay to check your bills.