debit order to be amended to the convenient date

I called the call center to amend my debit order date to be on the last date of the month from the 26 to month end. But they refuse to help me. They want me to fax the letter that shows the employment as if I am doing the new application. I told them that its not like I am doing a new application I just don't want my debit orders to bounce back I changed them with all other companies and Nedbank is the one company that is giving me a problem. If they want to double debit my account on the month end because the 3 days period that I want them to ament I don't mind. But I keep my profile clear hence I don't want any returned debits, so if they wanna keep the current date how are they going to find the money because I get paid on month ends. I'm being a responsible customer to take my airtime and call them so that they can get their money on the correct date. Its my 1st time doing business with Nedbank and if this is how they operate I'm not sure if I will like to do more business. I even reffered 2 colleagues of mine but I regret that. All I want is for them to amend the date which will only make a difference of 3 days in a same month and if they can't then I will pay them offand move