Terrible customer service

I went to this shop just to buy 2 black markers which would have cost me R18 and some change but the cashier told me that they don't swipe cards for less than R50. So I asked them to show me where in the banking procedure and Consumer Act does it state that there is a limit regulating the minimum amount for card payments. Well they couldn't show me anything but called the manager who was arrogant to the point where he was just pointing me to the door and telling me that he makes the rules and he doesn't care what I say and I must just leave the shop.