Frustrated

I am extremely frustrated with the service. I have been billed at R1458 for a R140 contract with a limit of R500. I called in for an explanation of this and the agent couldn't even explain properly where is the extra money coming from. Firstly I am being billed because I chose to make a cash payment within the 10 days of the debit order being processed which was never communicated to me. Also my February amount was initially R620 but you guys decided to debit R117 which is now becoming my problem. I can not afford to pay this R1458 once off. I the agent said I have an option of paying R502 for the 1st of April but this is not possible because the debit order amount will go off automatically.. I need feedback now. this is ridiculous and I can not pay such a big amount for a blady data contract.