FNB - Item paid no funds - hidden costs

So a couple of days ago I notices a huge transaction amount of about R360.00 added to my FNB gold cheque transaction list with the reference, ITEM PAID NO FUNDS. A couple of days later, another transaction with the same reference to the amount of R45.00 was also added. Upon calling the customer care line, the feedback I received was that this was a "service" offered to clients at an additional cost. I do not remember agreeing to any such service, nor is it listed on my agreement when opening up this account. FNB failed to provide me with the option of going over my available balance and informing me that by doing so I would incur huge service fees, or not to continue and use an alternative means of payment. As a side note, I have multiple products with FNB all of which would have been a better option to use.