Charged Hon fee

I was charged 3x hon fee when I queried it via the call centre a lady informed me there was not enough money in my account. After I made the purchases I received an sms to say the money was deducted why is the money not being deducted immediately and now I am charged round about R500. If I receive an sms that says the money has gone off my account I do not understand why I should be held liable for this. My account is now in a negative balance I have decided that I will rather pay it off because this only seems to happen with nedbank clients and I can not afford this