Item paid with no funds

I've heard stories of fnb robbing their clients with over charging,now I am proof of that. They have discontinued branch land lines and introduced app calling so that they can't be reached and branch consultants are clueless. I was charged R245.00 for Item bought with no funds which is not true.as I have a banking app you can see how much funds you have before transacting..how can 1 buy while there arr no funds. I am done with their services closing both my business and personal acc