Useless Town Square Spar Richardsbay

Went into this spar and tried to make a purchase which came to R19.98 and they would not allow me to swipe my card as apparently it's store policy that no transaction under R20 can be swiped for. This so called policy is not displayed anywhere in the store. Is this even legal? We are in the 21st century, people don't carry cash as it's just too risky. I spoke to the manager who was not interested. The local ****za shop down the road doesn't have any restrictions on cards and they do a fraction of the turnover of a super spar.