I keep right on walking. It is the duty of the STORE, not me, to make sure I pay enough for the services or products they provide.
I bought a car back in October of 2000. As part of the deal, they installed a 6-disk CD changer which I had agreed to pay for. I made the deal on a Monday. They were going to deliver the car on Friday (Halloween). I went in on Thursday night to see if they wanted to go over the paperwork one more time. I was told it was all set and everything was fine. No need to review anything. On Friday afternoon they couldn't get me out of there quick enough. Just sign these papers, take the keys and get the hell out of there. On Monday they realized they'd never added the price of the CD changer into the total price of the car (I hadn't realized it either). They TRIED to guilt me into coming in and cutting them a check for $450 to pay for the CD changer. I told them no way. Under CT Law (where I was living at the time) once they signed that contract, it was legally binding. Their mistake, their loss.