FNB ITEM PAID NO FUNDS I.E. FNB *****ING MORE OF MY *insert unsavoury word here* MONEY

FNB constantly charges me amounts ranging from R45 to R180 (totaling almost R400) in the past week. The reference is item paid no funds with the same date (21/07). I've tried to query and I got the same automated response.

What I don't understand is: #1 why is money still leaving my account days after purchases were made if my balance is nil by then? Surely if there are no funds the debit should decline as it normally does? (that we get charged R8.50 for). So when does FNB decide when to decline purchases and when not to??? #2 why am I being charged for the actual purchases made on a nil balance (it goes into the the negatives) and then charged again for not having funds to pay for that same item?? Another item not paid went off for 21/07 and I've definitely had funds in my account.

Are the exorbitant banking fees not enough for FNB?