Tracker keeping quite when unit is paid off

I had a Tracker unit installed in my vehicle three years ago, so the unit should be paid off now, and I should only pay for the service. Last month I received an INCREASE, not a decrease. I called to say that I've been there 3 years, unit should be paid off. They said fine, they will reduce my invoice from R280 to R208. But only in December. I said but the unit was paid off in August. They said no, its my obligation to notify them to reduce the installment, not theirs. How many people actually check that? Tracker knowingly take money that is not due to them