Double charged at Crocs waterfront

On the 27th of October I purchased a few items from the Crocs store in Waterfront Cape Town. A week later I realised that 2 transactions went off my bank account when I was there on the 27th one was for the items I purchased the other a mystery. I went back to the store on 14 November to show my bank statement and the recipient of what I purchased. The shift supervisor advised he would investigate and let me know. A week later I got a call saying I was over charged and I must come back in the store for a refund. The amount I was overcharged is R400, they can't explain how or why my card was used for that transaction. I dont frequent the waterfront often the trip alone there cost me R400.00 now I need to spend another R400. In total I would have spent R800.00 to get back R400.00 a mistake the store has made. The supervisor advised he will call me back to advise a alternative solution. I have received no communication since. As a consumer I can not be expected to be out of pocket. Funds was taken out of my account in store without my permission. There should be a explanation for it. Once the store concluded I was overcharged why is it that I need to spend more time and money and time to get it rectified.