ABSA CREDIT CARD IS ANNOYING

I have took a credit card with Absa with the impression that I will use it to improve my credit score. I have been paying my expensis with and repay it in full before the statement date. I always pay more than the credit limit to ensure that the R55 monthly fee is taken care of and avoid being less than the R20 000 limit. last month I paid up the credit card R20 100 and I was charged interest of R94.60 and budget installment of R139.43. I have refered this matter to their dispute resolution unit and they responded with no solution instead they justify the interest for over two months statement period while I having the account paid up every 30 days. I have decided to discontinue having their service it is very unpleasant with incompetence.