Wrong information at sales pitch, billed twice

I sold my car and bought a another car. The lady whom I spoke to said they have a special currently running where I don't pay any cancellation fee's or anything. It will be a new contract.<br> What she DID NOT take in consideration is that apparently the invoice for the next month in advance gets generated on the 15th of each month. So 2 weeks after I spoke to her they came and do the installation into my car 31 October. So obviously I get billed from that day.<br> Myself being the customer is now running a new bill + the \bill in advance"for my old car for November even though my old car is sold.<br> Now 1 December I get an invoice of double the normal amount. So I paid for my new device AND for the last month (month in advance) for my old car which I don't have.<br> <br> My issue is"