If you're using Preorder Pro + Back in Stock and customers are seeing an empty cart after adding a preorder item, check that Continue selling when out of stock is enabled.
Here’s how:
In your Shopify admin, go to the product page.
Click Edit on the variant you want to update.
In the Inventory section, check the box for Continue selling when out of stock.
Click Save.
After updating, it may take a few minutes for the change to take effect. Ask your customer to refresh the page or clear their cart before trying again.