Paid Up letter not being issued + Charged interest on a settled credit card account

My annoyance with Capitec has to be on another level.....I have been waiting for a paid up letter for a whole THREE months since settling the account. I then get told two weeks back that I still have an outstanding amount, which is alarming as on the same day I requested the settlement letter, I paid the exact amount....it beats me how an account can accumulate interest when settled. But nevertheless, I decided to pay the extra amount out of frustration and to get my paid up letter and cut ties with them and I sent this proof of payment for the additional amount.....Till today, I still don't have the paid up letter and still being sent from pillar to post with long expensive calls which prove to be fruitless. I am extremely annoyed and disappointed. The level of dishonesty is appalling and its actually day light *******. But I just want the paid up letter, keep the extra amounts being made up.