Return policy- non compliance of return policy

I'm so disappointed by the service I got from identity store , I bought an item online on the 1st of September which was 50% promotion for 5 days I got it on the 5th on the 9th I went to Mall of Thembisa to exchange for a bigger size I was told the promotion is over and I have to pay the difference mind you their return policy says refund and exchange are free within 30 days . I decided to contact the customer service I got the same response saying 30 days return policy. I went back to the store on the 16 th September I was told the same thing that I have to pay the difference the response from their customer service department is not what they know . I said okay let do the return guess what they have refunded the money and scan the item as a new purchase not as an exchange i asked why they don't do the return instead of the refund and they say thats how their system works they have refund first . I dont know but this is false information that they are providing, I believe that if I'm returning for the same item I dont have to pay any difference within their 30 days return policy. If that's how customers are treated we are ****** and they are not compiling with the South African Act and their return policy when it come to returns 😒 🙄. They even have the audacity to tell me to open an account with them after the service I got 😒 yoh never. I'm so annoyed by their return service and I have to pay the difference for something I bought .