Incorrect Billing and Payment Deduction

I app**** for a capped cell c data contract that was said to be of a monthly payment of R99, and last month I was shocked by a bill from cell c that said I should pay them R849 and I still don't understand why they billed me R849 but I paid it for the sake of protecting my self from arrears etc, and this morning they ran another debit of the same amount and I now realise that they are taking an advantage of me. So I would want them to bill me as per agreement between me and cell c.