Unpaid item

I am so disgusted with FNB,the way they rob clients is not fair.its day light *******. They charged me with banking cost at the end of the month,that I understand but on the 3rd of October they charged me with another R135 for an item paid no funds. So how did they come to this. This unpaid item that they claim was unpaid was something I bought while there was money in my account,so how could this be a unpaid item. This is the 3rd of 4th time I had this problem with them.