Charged interest and service fees on a paid up account.

I have been charged R60 on a account I paid in full. When I called the call center I was told that this is because I did not call in to get a settlement amount. I feel this is unfair and was never explained to me when I signed up for the account. I paid it inful with all the extra bull charges that gets associated with it per month now they want to add extra bull charges just for the fun of it... Gaan vlieg man