Wrongfully Charged by FNB (First National Bank)

I own a 'no transactional fees' FNB account. What this means is that I pay the bank R99 a month so that I'm never charged a transactional fee whenever I use my card for purchases. This is exactly how it is advertised; pay R99 a month and swipe as much as you like with zero banking fees!

Despite this claim, I made a purchase on the 25th of March from a local shop and the transaction went through with zero hassles as it usually has every month before that. However, to my dismay, I was notified by sms that I had been charged a large transactional fee on the 30th of March for the very same purchase I made on the 26th of March. Their reasoning for this is that said local shop "banked late" and now I must pay a transactional fee even though my account is a 'zero banking fees' account.

First of all, I don't owe the shop any money because FNB reserved the purchase amount from my account the very same day and moment (and instant) I swiped my card. When said shop then 'banked late' on the 30th of March, they received their money because it had been reserved in my account and all was settled. I don't owe them a cent. So why is it then that FNB is charging me an exorbitant banking fee (+R100) for a single transaction that I settled as usual? Is my account not a 'zero banking fees' account?

How could a professional institution approve such a ridiculous excuse, how could they blame me for some random shop banking at a later date than they should have, and how could they insult me even further by charging me fees for another bank's mistake even though I have a 'no banking fees account'? Is this not ********** behaviour? Where in the terms & conditions, and where in my account policy does it state that my 'zero banking fees' account is subject to exorbitant transactional fees?

Banking and Inconsistencies should never accompany each other, that is a fundament for *****. And before I change banks I demand to have this issue sorted.