Do you have any idea how annoying it is to small business owners when people have this mentality? "Call and threaten them to get some money!" First of all, calling the BBB will do absolutely nothing. A few complaints per year improve a business' rating, because it shows that the business is operating within their range determined by the industry average complaints. Second, I know the OP may be mad, but you should not call and threaten them. The first way to tell someone is talking out their a** is when they immediately make threats. "I will call my lawyer" is the best one (really? You're going to pay a lawyer $50/hour to get the $50 you lost?) You should simply call and politely ask for fair compensation after describing the issue.
Sorry for my rant, just hate people who feel the need to immediately resort to threatening small business owners. Best wishes in your situation.