Shopify Functions allow us to write custom logic that runs on Shopify's servers during checkout. We can use these to enforce business rules, such as validating address fields and blocking checkout if the address is invalid.

Let's create a validation function that checks the address fields (e.g., street, city, postal code) and returns an error if the data does not meet our criteria.

If the validation fails, the checkout process is blocked, and the customer is shown an error message.