Zip Code Validator by ENS Enterprises is one of a family of Shopify apps by the same developer but none of the others are rated quite so highly as this by Shopify merchants. The app has maintained close to a five-star rating over the past few years.

This app doesn’t actually check shipping addresses to correct errors before shipping. Instead, it allows customers to check a product’s availability using a zip code and is designed for merchants who can fulfil orders only to specific areas.