Adding to cart not working for Product Designer

When you go to our product page: <a href="https://heavymetal.sherlockdesigns.com.au/designer/?product_id=5448">https://heavymetal.sherlockdesigns.com.au/designer/?product_id=5448</a>

Steps to replicate:

You customize the selection,
You press to generate an image
You press add to cart
The pop up says item added to cart
Go to cart nothing is there.