Contact them and say that you will be contacting the state attorney general if this is not resolved. Make good on the promise. Businesses are terriified of the AG. I had an apt complex cheat me. I told them I was going to contact the AG. They said fine. I did. The AG just sent them a letter saying they were starting an investigation and the complex immediately cut me a check for the disputed amount.

Also, don't worry too much about the "past due" notation. Our local hospital sends ALL bills out stating that they are past due. It is apparently a way to motivate you to pay quickly?

Good luck!
EJ