Interest charged on fully paid account

I made a few purchases and have always paid extra on my account to try get it paid faster.On my statement date 08/06/2015 my balance was R355.51 which I paid in full.Thinking the account was settled as I paid the full statement balance, I was rather suprised to see a statement with an amount the following month. Statement dated 08/07/2015 had a balance of R8.07 for interest. I called Homechoice to query this and was told I was incorrectly charged and the account would be rectified.Statement dated 08/08/2015 now says I have a balance of R37.76 for more interest that I have been charged. I emailed all 3 statements to customer service requesteing the matter be looked into and resolved as I have paid the account in full only to receive an email stating I need to pay the settlement of the account should I wish to pay it up and I should request the current settlement amount value as this is the reason the account will still incur interest.How is this possible if my I have paid the full statement amount and was previously told I was incorrectly billed for the interest as the account is paid in full?Does anyone in this company know what they are doing or are you just trying to rob clients?