Checkout extensibility, explained for merchants

For years, checkout was customized by editing a theme file, checkout.liquid, and by pasting tracking code into “additional scripts” on the order status page. That model is over. Checkout extensibility is what replaced it: customizations come from apps, run in places Shopify controls, and survive checkout updates without breaking.

For a merchant, the practical questions are what each part does, which plan it needs, and what on your store may have silently stopped working.

What ended, and when

• checkout.liquid is unsupported for the Information, Shipping and Payment steps.
• checkout.liquid and additional scripts were sunset on the Thank you and Order status pages on August 28, 2025.
• Script tags on those pages were sunset on August 28, 2025 for Plus stores, and on August 26, 2026 for stores on other plans.

The last date matters now. If a non-Plus store relied on a script tag for a conversion pixel, a survey or an affiliate tag on the order status page, it stopped running at the end of August 2026. Shopify’s guidance is that stores still using checkout.liquid on the Thank you and Order status pages must upgrade to Shopify Extensions in Checkout.

The parts, in merchant terms

Shopify’s developer documentation names four customization options, plus branding:

The first two are where most custom work happens: UI extensions for what the buyer sees, Shopify Functions for what checkout does. One of the most useful Functions, validation, has its own guide.

Two of these you mostly install rather than build. Payments extensions come from payment providers, because only approved Partners can build them. Web pixels usually come from the analytics or ad platform’s own Shopify app. The parts a store commissions are UI extensions and Functions.

What your plan allows

Shopify calls the first column “standard” customization and the Plus-only items “advanced”. A store below Plus can still change a lot: how checkout looks, what appears after purchase, and, through App Store apps, the discount, delivery, payment and validation logic.

How the checkout editor works

Once an app with a checkout UI extension is installed, you add it from the checkout editor: open the section, click Add block, choose the app, and configure its settings. Blocks can be dragged to another area. A single location holds at most three apps.

Some apps behave differently by plan. The Checkout Blocks app, for example, is available with some feature restrictions on Basic and higher and in full on Plus, and the Help Center notes that some apps are available only to certain Plus stores. Check an app’s plan notes before you design around it.

Extensions can only use the components and APIs Shopify exposes, not arbitrary HTML or CSS, and they don’t see sensitive payment information. That is the trade-off against checkout.liquid: less freedom, but customizations no longer break when Shopify updates checkout.

Where the old customizations go

Most legacy checkout code falls into a few patterns. This is where each one lives now:

One pattern has no direct replacement: arbitrary markup and styling. Extensions use Shopify’s components only, and branding goes through settings. If a legacy design depended on custom HTML inside checkout, expect to rethink it rather than port it.

Three common misreadings

• “Checkout extensibility is a Plus feature.” Parts of it are. Branding in the editor, post-purchase page apps and App Store apps with Functions work on lower plans.
• “Functions and UI extensions are the same thing.” They are separate tools that often ship in the same app. One adds interface; the other changes logic on Shopify’s servers.
• “Tracking can go back in with a script tag.” On the Thank you and Order status pages, script tags are sunset for every plan as of August 26, 2026. Pixels are the supported route.

What to check on your store now

1. Open the order status page settings and list anything that used to live in additional scripts or script tags: pixels, surveys, affiliate code.
2. For each tracking item, check whether the vendor offers a Shopify app with a web pixel. Replace the old script with it.
3. For each piece of visible checkout content, decide whether it belongs on the Thank you or Order status page (any plan) or inside checkout steps (Plus).
4. For any checkout logic, such as discounts or hidden options, find the Function-based app or build that replaces it. If it used to be a Shopify Script, it stopped running on June 30, 2026.
5. Place a test order and confirm tracking, messages and rules behave as expected.

Where Lintel fits

Lintel builds the custom parts of checkout extensibility: Shopify Functions for logic, and checkout UI extensions inside a custom app when no App Store app fits. For a store with legacy checkout code, the first step is the audit above, so the rebuild covers what actually mattered.