Poor customer service, refused to exchange

I am very disheartened about the treatment that I got from Sandton City. I bought a t-shirt online and it happened to be a big size. On Friday, 26th of September I went to Sandton to exchange the t-shirt and was assisted by a lady whose name is Mpho. On Sunday when I was putting on the T-shirt, I realised that it was damaged. I then went back to Sandton to exchange the Item, I was then assisted by a lady called Denise, which I believe is the Store Manager who advised that they will have to do investigations regarding the damage because She certain that it can't be their fault as the stock gets checked by 3 people before it gets to the floor. I was shocked , but fine I let them coz I believed that they were just doing their job. Denise said that they will have to call the supplier and they will also have to check the footage of when I came to make and exchange. I then asked her how often is she going to give me feedback and She said every time she gets new feedback, she will update me. It was Sunday when I came, but the whole day Monday I never received any feedback. Tuesday I still didn't receive any feedback. Only when I called the store to ask about the feedback, which I even had to call twice coz the first time I called the lady made me hold the phone without anyone coming to attend to me. I had to drop the call and call again then the guy who answered the phone was kind enough the get them to call me. I was so disappointed that a whole store manager couldn't adhere to sunset principles, or I don't know if it's because I was just looked down on because of the way I looked. When Mpho finally got on the phone to give me the "feedback" that she was sitting on the whole time, she advised that the supplier claimed that there is a certain code that is not there on the t-shirt, of which I would not know of as a client. When I asked if they had checked the footage as she advised , she said that further investigations need to be done which include the checking of the footage. Then today which is Wednesday, she called again and told me that they will not be able to exchange the t-shirt as they strongly believe that the error is not on their side because the stock gets verified by different people before it's gets to the floor. So I asked Mpho if she supervises everyone who verifies the stock and she said yes. I then asked her that if they truly verify everything then she should have checked the t-shirt if everything is in order, she would have picked up that the t-shirt she gave me didn't have the code that they claim that my t-shirt didn't have. She would have also seen that the item is damaged even before she gave it to me. How is she so sure that the t-shirt she gave me is in good condition when she didn't check it upon the exchange? What I am also asking myself is that is the stock "verified by 3 people" on the floor daily that they are so sure that it's in good condition? I wonder if they even checked the footage because if they did, they would have seen that she didn't even check nothing on the t-shirt on the day I came to exchange. She didn't even give me the receipt of exchange on that day. So why should I be the one suffering while didn't even ensure that she gave me the t-shirt in good condition. I am deeply disappointed that such a big brand would such poor customer service. What I know is that I will not pay my hard earned money for a t-shirt and not be able to use because people don't want to take accountability. Their staff is without remorse and should be well trained on standards of good customer service.