They dont keep their promise

Hie. Last month i had not enough money to pay my account. there is a lady who phoned me to arrange with me. she said to me she has put it in your system that the minimum amount i can pay will be R300. i paid 320 . To my suprise i am now being charger R20 for a col.letter fee whereas we agreed on the money that i must pay. im getting smses also. is that how u do business. i have never been in arrears ever since i opened my account. is this how you treat your customers. not happy at all.