You are right in being upset about how your were treated...even more so than the math error. For a small business or any business really, it is waayyy more economical for them to "keep" a customer than to find a new customer. As a former manager...I would suggest a politely worded letter, stating the FACTS of the exchange and suggest what will make you happy at this point...money back?...an apology???..a coupon??? Whatever it is, they should be happy to talk with you about it and come to a solution. If not...I would definetly take my business elsewhere and be happy to spread the word about the poor service.
I once had a problem at a cleaners...they ruined a suit and didn't want to pay for it. I told them nicely that their decision was no problem, I would just print up fliers about the issue and put one in EVERY mailbox within a 1 mile radius of their store. They immediately paid for my suit. :roll:
Good luck...let us know what you decide to do.