Unauthorised deduction

My tracker device was newly installed for me on the 29th December and upon activation the agent told me that I would only be paying for January only because December was technically over. To my surprise in January I was double deducted for Dec and January. I really feel it is unfair because if I was advised otherwise I would tell them to fit the device in Jan. It's unfair to pay a whole months contribution whereas there were just a few days left. I need to be refunded because I was told that I would only start paying for January. Really you can't say one thing and do another. I'm not to be blamed for your internal processes because when I called the agent said the people who sign us up know nothing about how accounts are handled. Please resolve this matter urgently!