Overcharged by R920

My instalment due for March was R1538 and some change, I paid R1540 on 26 March. I always pay a little extra. On 7 April, I get an SMS to say I need to pay my account to avoid being a late payer. I call customer care and go through to financial services. The consultant says I have an amt of R814.83 outstanding although I have paid the full Amt plus a little extra already. She can't understand or explain to me what happened. I get my Account statement now and there's an outstanding amount of R814.83 plus a late payment charge of R104.79. How does Edgars just add on close to R1000 for no reason as I've been paying more than I owe every month and who do I speak to that could actually help in this matter, do I need to get attorneys involved as the consultants from financial services do not know what is going on with Accounts! I am fed up to the point where I am thinking about paying off my account and closing it!