Overcharging on credit card

In May this year, I sent an email to Standard Bank credit card division, inquiring about how they calculated FINANCE CHARGE on client accounts. The reason was simply this. Between June 2020 and May 2021, I never used the card as a payment method. Instead, I was paying off the debit balance every month during those 12 months. During that period, the interest rate remained the same. However, for some strange reason, this FINANCE CHARGE kept going up.

Standard Bank's first response was to dismiss my inquiry by telling me that if that is on my invoice/statement, then it must be correct. Since this was very annoying, I insisted that they show me how the charge is calculated. After days of trying to deflect my inquiry, they came back and told me the bank does not keep information for more that 3 months!! Yes, I have an email that states that. I sent them a sheet with all the charges for that 12-month period and then they kept sending calculations of a different charge. When I pointed that out, the stopped responding to my inquiry, even though they know it is unresolved.

So, all bank division staff do not know how to calculate a charge? I even offered to calculated it, which I'm perfectly capable of when given all parameters such as the interest days, and the outstanding balance. They would not take up my offer and only told me a credit card account has two balances, depending on how the card was used for. But which they would not provide the actual amounts.

I believe they are miscalculating the charges. Please help.