Billing nightmare on Contract

I've called Telkom in April to change my debit order date to the last day of the month to coincide with my salary. The lady I spoke with confirmed that it was amended. My debit order went off in May, June and July on the 25th instead of the last day. At that point there was insufficient funds and Telkom charged me a fee of R200 for not having those funds available for them! The call centre agent stated that i need to go to the store to sort this out, for which i have to take leave to resolve! This is highly unfair and unnecessary. Telkom is the worst service provider ever!