Outrageously overcharged bank fees

I have a business account with FNB and my bank charges are charged at R75 a month and average +- R400 a month. However, this month it all came to R992!! When querying it, I am told that when the transaction is processed by the merchant at a later stage, the funds are no longer available which puts the account in arrears. If this is the case, then why are the funds immediately deducted from my available balance? This cannot be correct as that would mean I spent double as much as I would have. Terrible service and ********** fees.