Disconnect at the end of my business---Horrid fees for disconnecting

I sold my medical practice in May and disconnected my Comcast Business servvices on June 18.  After an inordinantly long phone call to the Comcast "Rentention Team," I finally got the message through to the agent that, no, I really didn't want to continue service because I was retiring and someone else was taking over the practice.  (The new practice has its own provider for phones and internet).

The agent then sends me a form that asks me to recognize that I will continue to be billed for another 60 days.....WHAT???  My business was FINISHED.  There is no need for an additional 60 days of service.  So I signed the form but also removed the 60 days stipulation.

I made the assumption that this was the end of it.  But then, without receiving any further communication from Comcast, I received a letter from a Cleveland collections agency for the ~$137 they felt I still owed!!!

Further, I have reviewed the original documents that I were signed 1/12/12.  No mention of any disconnect fees other than early termination---we were >5 years past that.

I don't understand how a company can arbitrarily add charges at the conclusion of a contract without telling the customer beforehand.  Pretty ridiculous.