More comcast service problems - or - why don't they get it?

Comcast problems; anyone have this problem with the helpful folks at comcast? I just had new service installed in a shore home, and got a written estimate from comcast on the price. The installer came, and said he had to run a second line (not sure what that means, but I said OK). He said that there would be no additional charge for this. His written bill said that there was no additional charge for this as well. The bill in the mail had a $75 bill additionally for a second line. When calling Comcast I was told that there was no one available to speak to with the authority to change the bill. A supervisor said this was "corporate's decision" to bill this, and she didn't have the authority to change it, and she would not give me the name or number of anyone to talk to. I did get an address to write to, but no one to address it to. Worse, she was rude and arrogant about the whole issue. Any thoughts on a good, public way to address this, and why does this monopolistic company continue to treat us like serfs? We need some competition soon, or it will only get worse.