Item paid charge

I am really not happy with the fact that im being charged R75 for "items paid" and the response i am getting from the private bankers when trying to dispute the charge,is that the merchant banked later when there was no sufficient funds in my account.

Why should i be charged just because the merchant banked later??? I rectified my account immediately after realising it was overdrawn,before you even sent me an sms, so please rectify this...asseblief