Do you have "small claims court" in the U.S.? If so take them to court. Worst case is you will pay the $50 or so they say you owe. Best part is they would have to send a Lawyer. Of course they would never do that for that small amount and so I bet you will get a settlement to reduce the bill to $0.
In Montreal we can do that. I've used it and I have been a witness for a customer of mine that was accused of not paying a $1500.00 bill to a web designer. He ended up paying $750.00. But that is here in Montreal where we can use small claims (up to $6,000) without having to go to a lawyer.
Up here for small amounts, we write our own "Without Prejudice" letters and send them registered mail. I am not sure what the law says you can or cannot do in the U.S. when it comes to this kind of argument between 2 parties.
Maybe you should call a law firm and ask them to refer you to a lawyer that does Pro-bono work. You may be able to get some free advise at least.
In any case good luck. At least its under $100 so its not a heart attack amount!
Jim