Collections please help

I moved out of an apartment in June of 2009. Canceled all services to the said apartment. However, in December 2009 I noticed that I was still being billed for my gas service at the old apartment. So i contacted the company, and informed them that I had canceled service in June.

To this they disagreed, and said that I had never canceled my service. I have no proof to show otherwise except for the proof that I moved out of the apartment. I say fine and instead open a dispute with my bank. They finally agreed to refund me my money for November and December. Mainly because they were already charging the new residents in the apartment during this time.

However, yesterday we got a call from a collections agent with the company stating that they have now reported it to the credit bureaus. They indicate they had been mailing items to our previous address. The truth is that we never received anything from them even though we have received all other mail that was sent to our old address as part of the forward services of the USPS.

So what to do? It's relatively a small amount of money ($30). Should I just pay and work with them to clear my credit history? How will this negatively impact my credit score? I was in the high 700's prior to this.