Incorrect Billing

I got a quote from ADT to install an alarm system in my new house from Charles Louis. He came and we talked about my need and the price of the system. When we agreed on what was to be installed I signed off on the package and was told that the amount will be debited from my account. After the system was installed I got the document regarding the alarm system and noticed the amount payable had almost doubled so I phoned Charles to enquire about this and he told me the system calculates each component separately and therefore the amount is different to the package deals and not to worry as this is just system error. I accepted this explanation and signed the work order for the installation. When the bill arrived it still had the incorrect amount and after talking and emailing Charles as well as phoning ADT head office nothing has happened and I'm still receiving a bill with the incorrect amount but now it's over 90 days overdue. I refuse to pay the balance as this was not the originally agreed upon amount.