You can choose to show a warning when a customer adds both preorder and non-preorder items to their cart.
To manage this:
In the app, click Settings.
Look for Show mixed cart warning. This is turned on by default. To turn it off, uncheck the box.
You can also update the text for:
Warning title
Warning message
Button text
This message appears as a pop-up on the cart page. Click Save when you're done.