Shopify Checkout UI Extension Development: A Step-by-Step Guide (2026)

Short summary

This guide walks through building a Shopify checkout UI extension that adds a delivery-instructions text field, covering target placement, reading and writing checkout data via the shopify object, local testing, and deployment. It clarifies when to use UI extensions versus Shopify Functions, noting that Scripts stopped executing in June 2026 and must migrate to Functions. The article includes practical advice on API version pinning and common failure points in production.

• •Checkout UI extensions are sandboxed React/Preact components rendered in fixed checkout slots
• •UI extensions handle visible UI; Shopify Functions handle server-side pricing, shipping, and payment logic
• •Shopify Scripts stopped executing June 2026 — migrate legacy logic to Functions, not UI extensions

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