FNB prepaid electricity purchase arrears amount

Everytime i buy prepaid electricity through FNB app, it charges me an arrears amount. How can i be in arrears when I'm buying prepaid electricity. The amount at times goes up to R25. Fnb does not have an answer to this and asks you to contact the municipality when FNB itself is the one charging you this amount. FNB has found another way of *****ing up their clients. FNB should change their tagline from how can we help you to How can we ***** you.