Items Paid no Funds

Item paid no funds. I made a purchase online using my FNB cheque account, the money was deducted same day from my account, a few days later I'm seeing the item paid no funds of R180, I called the bank and the operator told me the money was only received but processed later after I had no funds in my account which resulted on this. I do not understand this. The money was deducted from my available balance of which I couldn't and didn't use it how come that I have to pay items no funds if the bank process payments later, this is day light ******* I want my money back please.