Why Isn’t the Mixed Cart Warning Working in PreOrder Now?
If the Mixed Cart Warning in PreOrder Now isn’t working as expected, the most common reason is that your theme’s AJAX cart or add-to-cart script doesn’t support Shopify’s line-item properties.
Cause: Theme Doesn’t Support Line-Item Properties
PreOrder Now uses a cart label to tag preorder items when they're added to the cart. This label is a line-item property, and it’s how the app detects if a preorder item is in the cart.
If your cart or drawer doesn’t show this label when a preorder product is added, your theme likely doesn’t support Shopify’s line-item properties via its AJAX cart.
How to Fix It
Contact your theme developer and ask them to add support for Shopify’s line-item property feature in the AJAX cart / add-to-cart script.
You can refer them to this Shopify guide: Build Shopify themes
Once the theme supports line-item properties, the Mixed Cart Warning will start working properly.
Tip: Many other apps also use line-item properties for customizing products, so enabling this support benefits more than just PreOrder Now.