Shocking service / possible theft at superbalist.com

I ordered 2 of the same dresses in different colours off superbalist. They were delivered, I tried them on and they were too big. I logged an exchange online the same day and packed both the dresses into the box they came in. The driver came to collect the box with both of the dresses inside. Superbalist is saying that only one dress was returned and refusing to give me a refund / credit / exchange for one of the dresses which I have paid for. I have 2 witnesses at The Gone Store High Constantia when the order was collected ( the store manager and the marketing manager) This has been an ongoing issue for nearly 3 weeks and superbalist are simply refusing to take responsibility. How are we as custemors meant to be protected from this type of thing happening in the future?