When a customer attempts to checkout with a restricted product and address combination, they are prevented from completing the order and shown an error message similar to the following:

Taking a closer look at the error message itself:

we see that, aside from the list of affected products, the error message has two parts: a title ("Address Error") and a body ("We are unable to..."). Within the app, there is a page called Checkout Messages where you can modify the title and body of the error messages that appear in your store's checkout.

You can navigate to the Checkout Messages page by clicking the Checkout messages link in the app's navigation menu:

On the Checkout Messages page, you will see two sections: ZIP Code Error and Province Error.

When a customer is blocked by a rule that targets ZIP/postal codes, the ZIP Code Error text will be used to create the error, while customers blocked by a rule targeting states/provinces will see the text from the Province Error. Error messages for customers who are blocked by both ZIP code and province rules use the Province Error text.

The help text beneath the text inputs shows the default text that will be used when no custom values are set, and the preview at the bottom of each section shows how the error messages will look in checkout.

Note, the list of product titles at the bottom of each preview is just placeholder text, and will be replaced by the affected product titles during an actual checkout.