Bad service with FNB - credit card

This is regarding FNB private client credit card and service.

I made a full payment in Feb two days before the payment date, and on the payment I still got debited the minimum payment in my cheque account. I was told my the service desk that minimum payment will be debited if I did not pay back my credit card within 5 days of the deadline....fine

So in March, I created two scheduled transfers from my savings account, the minimum payment into my cheque account for debit, and the rest of the full payment into my credit card account directly....seemed like a good idea at the time

Coming to a day before the payment date in March...my rest of balance (R30k+) was transferred successfully....but the minimum (R1.8k) was not debited on the payment date.

Therefore, I had to manually transfer the amount a day after to the payment date (10% of the total payment)

A few days later I get the credit card bill which showed both payments, but I get an interest of R850+.... I asked for an interest breakdown, with the service desk and my private banker...with no response. The only cash advance I used was something like R650+ which is included in the R1.8k (10% of the total payment.)

Its been a week and no one has given me any answers yet as to why R1800 owed (not even a late payment because I paid 90% before the end date) would equal R850 of interest??? With an annual interest of 13.25%, I really want to know how it was calculated so I can avoid these things in future.

FNB is known as the bank with the best services in SA, but right now I'm wondering why I'm paying almost R400 a month for such bad service and unclear visibility...