Contract Price Not Honored

I signed up and receive service from Directv. The price I was quoted is not the price I am receiving for my service. I contacted them about the price difference and they claimed that was because I hadn't been receiving an "online" credit that would be applied to my bill.
I now have started receiving this credit but my bill is still not correct. Since March I now have been charged over $150 too much on my bills. When I addressed my concerns to them again I was told that my bill was correct and that if I didn't pay it in full by the due date I would also have to pay a late charge.
I have been paying the bills because I don't want anything negative against my credit and if I cancel I will be charged an outrageous amount for early cancellation. What can I do?

Based on what you say here, you could file suit against them (or at least threaten to do so) in small claims court or in regular court; filing in small claims court is easier and requires no attorney...you could try to collect all of the costs and damages that you suffered due to their behavior... You may also report them to your state attorney general's office.