Wrong Billing

My husband and I app**** for an explorer last year november. It has been delivered on time and we knew that it will start to bill us on the day of delivery. It conneceted few days after. the first month to pay was november and we were billed R4000 but DSTV said it was their mistake for adding interests. our payment is supposed to be price lock and extra view which is 700. W are tired of calling dstv every month for not fixing their error. we do not even enjoy our explorer because they keep on shutting the account saying we owe them of which we dont. Please help us resolve this quiry. my husbands id number is ********** 087080. Theire wrong billing is frustrating us. please help