Service fees/Interest on Zero balance

Edgars are now charging interest and service fees on purchases. I previously queried and they told me that if balance is zero, there will be no charge. On the 2nd March, my account balance was zero for a while. I purchased an item and put it in my account for tracking purposes. However, immediately paid the full item cost within a minute. Account balance goes back to zero on slip. Received a statement on the 10th March showing that service fees and interest have been allocated on my statement. This is a ******* and i regard this as *******. RCS processes are really destroying Edgars as it is most frustrating in their services once RCS took over.