Shopify Glossary Terms

Checkout Extensibility is Shopify's framework for customising the checkout experience on Shopify Plus stores. It replaced the legacy checkout.liquid approach with a suite of tools - Checkout UI Extensions, Shopify Functions, and the Checkout Branding API - that run in sandboxed environments within Shopify's infrastructure.

The migration is effectively done. Shopify deprecated checkout.liquid for the main checkout pages (information, shipping, payment) in August 2024, then shut down checkout.liquid, script tags, and Additional Scripts on Thank You and Order Status pages for Plus stores in August 2025. Auto-upgrades rolled out from January 2026. Non-Plus stores have until August 26, 2026 to complete the migration. Separately, Shopify Scripts (Ruby-based discount/shipping/payment logic) are being replaced by Shopify Functions, with a deadline of June 30, 2026.

What you can do with it: add custom fields (gift messages, delivery dates, VAT IDs), in-checkout upsells and cross-sells, post-purchase offers, trust badges, loyalty point redemption, branding customisation (colours, fonts, corner radius, spacing), and payment/shipping/discount logic via Shopify Functions. As of 2026, there are over 20 extension points across product information, shipping, payment, order summary, and Shop Pay. The Winter 2026 Editions introduced market-specific checkout customisation - letting you tailor checkout content and rules per region - and support for additional address fields in the checkout.

What you can't do: fundamentally change the checkout layout, inject arbitrary JavaScript, or override Shopify's payment processing. UI Extensions run in an isolated sandbox powered by Remote DOM, with no access to the checkout page HTML or sensitive payment data. Extension bundles are capped at 64KB. These constraints are intentional - they protect conversion rates, security, and upgrade compatibility.

The trade-off is less raw power for significantly more stability. Your checkout won't break because a third-party script conflicts with a Shopify update. One practical consequence: analytics tools that relied on checkout.liquid or GTM no longer work in the new sandbox. Conversion tracking now runs through Shopify's Web Pixel API using sandboxed iframes and analytics.subscribe() events. Read our full checkout customisation guide for what's actually possible.

Think: the framework that lets you customise checkout without ever breaking it.