Intrest Charges on a Zero Balance I refuse to pay

Ive paid my full outstanding balance on my TWT express card I'm still being charged intrest on a zero balance and insurance and service fee. I called in to get it sorted out to which someone said it was a mistake and proceeded to do nothing about it. i called in today again and told im liable for that R78 on zero balance, this is clearly ******** and and if its not resolved ill be taking it up with the NCR