Contracts payment

I have requested that Cell c debits my account for my contracts (2accounts) on the last day of the month however they have been debiting one on the 28th of each month and the other on the 01st. I called the call centre and asked the agent to move the dates to the 15th of each month as i will do an EFT and cancel the debits. This month i did an EFT into Cell c accounts and still i get debited for my contracts. I have been charged an additional R79 for late payment on one contract. Can someone please assist me as i am getting no joy speaking to the agents at the call centre.