Carol vs. Comcast

Comcast cablevision in flint michigan wont keep their promise. I signed up for a comcast bundle after being with att for 3 years. A sales rep. came to the door and offered me a good deal. She gave me a flyer with the price on it. It said that I get the triple bundle for 89.99 for a year and after the year is up it will be 109.99 for the next year. Now they have sent me a bill for 193.98 for the month!! I contacted the sales rep and she said someone must of made a mistake and she would fix it. Well, now she is avoiding my calls and comcast wont stand behind that flyer. I would like to write a letter but I dont know who to write it to. Can you help me out? Thanks