Takealot Wont Refund me for a return

I purchased a reflexology Sandals on the 9th Nov 2023.On the app they only had a size 5 so, I thought 1 size fits all. When i recieved it i tried it on too small. I placed it back in box and logged a return. Courier fetched it on the 13th Nov 23.Now 10 days later they say cant get a refund due to box being opened. So how am. I to know if it fits if i dnt open the box. The problem is they selling china stuff and sizes are not S. A sizes. Im very disappointed in takealot as this was my first purchase. Never again will O order from them