DISCOVERY CREDIT CARD

I owed R28 321.89 on my credit card payable by the 3rd July 2019. I paid the amount due on the 24th June 2019 (9 days before it was due), BUT unfortunately made a mistake and paid R28 231.89 (R90-00) short - finger problem - and now Discovery charges me R832.56 interest because the full amount was not paid. We have been clients for many years and this is the way we are treated - although the credit card gets settled in full every month. It took 3 calls and lots of time on the telephone to find out why I was charged the interest, and was told today that I made the error when paying the money and that is the reason.