ZARA return policy breach

Received a gift, tried to exchange the item for a different size. Still has the tag on it and in original packaging. After several trips, calls and conversations with managers, Zara refuses to assist as I dont have the receipt. We can supply proof of payment via sms and bank statements, they refuse This is against CPA S20 and they keep getting away with poor service and non refundable policies. How are they exempt from SA law>