Can FNB help or are merchants untouchable?

I went to purchase on Sunday 7/6/2015 at my local OK Grocer in Theunissen, and FNB MERCHANT. I was at the front counter where the product were, and the front counter lady ( a white lady with dark hair and purple / blue stripe in her her hair - she did not have a name-tag on) told me her manager said that because of BANK COSTS I am not allowed to buy anything under R20.<br> I told her as far as I know , bank costs on merchants are worked out by percentage and that a lower price on an item does not mean a bank higher cost. She still refused and I was forced to buy a 4 pack just over R20.<br> I told the lady that I am pretty sure that this is against the FICA Law of the Reserve Bank of South Africa and the Merchant agreement of all SA Banks. She told me out right that there was no such law ( as I was making it up) . I told her that I would bring her a copy of the law on Monday and she told me with a bad attitude to \BRING IT"as if the law does not exist. It was very humiliating because there were at least 10 people standing behind me in the queue .<br> She did not treat me nicely and never attempted to call a manager to help.<br> Will FNB help? I have a copy of the Merchant agreement: read point 7.."""