Debit Order

I have a contract for R99.00 with cartrack but they are always debit more money different amounts up to R400 and they do this even during the month and i really don't understand why and when i call them they keep transferring me from one department to another. Even if i was behind with payment why do they debit different amounts and during the month and not the same amount we agreed upon ??? i am very disappointed i wish i can cancel the policy...