Debit order date not changed as requested

I have instructed Discovery Bank to deduct their banking fees and interest on my card on the last day of the month. This is not my primary banking method and I don't check often, This month they've deducted on the 4th - again. This means that I go in overdraft and for this they report me to ITC. No warning. No mail. No courtesy call. This is regardless of the fact that I have set up a channel and card limit to build in a buffer. So Discovery bank's "Limit" means something different than what I understand. Do not call it a "Limit" if its not a limit. And I also do not care for your behavioural banking model. A limit is a limit. Please change your debit order date to the last day of the month as instructed and as you agreed to. I am sick and tired of large corporates doing what they want and getting away with it.