why is the closing balance keep on getting more?

I paid the closing balance on 15/12/20014 and they still send me letters that I own them money I can't understand how is that possible. I got a letter I from them in January 2015 that say that I own them R0.58 and then this month they say it is R29.58 I just don't understand how can the closing balance keep on getting more and I didn't buy anything from them since I paid them on 15/12/2014. I Ask the lady at Game Menlyn Park what the closing balance was so I paid that amount and on the slip it said to me I don't own them anything but then I got the letters from them saying I still own them money how is that possible . I am really unhappy with them. I don't mind paying them its just unfair why is the closing balance getting more each month.