Bad experience with Vodacom

My experience with Vodacom has not been very pleasant. I have a contract with them. In December they said i have missed a payment, fine, and they added a R100 more on my instalment. I paid the R100 more on my instalment on January using the self service terminal. In February when they sent me the instalment, they have added R200 more because i paid on the 24th and my debit is on the 25th. When I called to enquire about this they said i should have paid 7 days before my instalment. The instalment remains the same amount and they can’t do anything about that, nothing is going to change. I am treated like i have paid late even though i paid earlier.