Shopify Checkout Extensibility, Migration and Customization

The checkout is where the money is, and it is the part of Shopify you have the least room to change. Since checkout.liquid and Shopify Scripts were switched off, the only supported way to change it is checkout extensibility. We are the team you bring in to move your checkout onto it, and then to make it earn more.

Where checkout.liquid and Scripts stand now

Every deadline has passed. If a customization was not rebuilt in time, your store fell back to the default checkout and it is gone.

What changed	Date
checkout.liquid stopped working on the information, shipping and payment steps	August 13, 2024
Thank you and Order status pages upgraded on Shopify Plus, Additional Scripts made read-only	August 28, 2025
Shopify Scripts can no longer be edited or published	April 15, 2026
Shopify Scripts stop running on every store	June 30, 2026
Thank you and Order status pages upgraded on every other plan	August 26, 2026

Dates are from Shopify’s developer changelog and Help Center, checked in September 2026.

So the migration is no longer about beating a deadline. It is about getting back what quietly disappeared: a delivery message, a gift note field, a hidden payment method, a tiered discount, a tracking tag. Then doing it better than the old version did.

What moves where

Almost every old customization has a clear home on the new stack:

You used to have	It now lives in
Content, banners and fields added in checkout.liquid	Checkout UI extensions
CSS and layout changes in checkout.liquid	The checkout editor and Checkout Branding API
Line item Scripts (tiered pricing, BOGO, gifts)	Discount Functions, and Cart Transform for bundles
Shipping Scripts	Delivery Customization Functions
Payment Scripts	Payment Customization Functions
Additional Scripts and script tags on Thank you pages	Web pixels, plus UI extensions for anything visible
Checkout checks that blocked bad orders	Cart and Checkout Validation Functions

The mapping is clean. The work is in the detail, which is why we treat it as a rebuild, not a copy and paste.

Scope: what a checkout extensibility project covers

Every project is scoped to your store, but most include the same parts:

• Audit. We list every customization your checkout had or still needs, from your old theme files, Script code, Additional Scripts and past orders, and mark what is still worth having.
• Plan and fixed quote. Each item gets its new home (extension, Function, branding or pixel), an order of work, and a price and timeline you sign off before we build.
• Build. Custom UI extensions, Functions and branding, built in a development store with weekly demos so you see it working before it goes near your live checkout.
• Test. Real cart scenarios, including the awkward ones: large carts, stacked discounts, international addresses, B2B customers, and every payment method you offer.
• Launch and monitor. Changes go live one at a time with a rollback path, and we watch the first live orders and your analytics closely.

Deliverables

At the end of a project you have:

• Working checkout UI extensions, Functions and branding, deployed in an app on your store, with the code handed over.
• Merchant settings for copy and thresholds, so your team can change them without a developer.
• Tracking moved to web pixels and checked against real orders.
• A written record of what your checkout now does and where each rule lives.
• Clean, documented code, so you are never locked into one agency.

How long it takes

It depends on what you need rebuilt, which is exactly what the audit answers. One checkout block is a small job. A full migration with several extensions, Functions and tracking is a bigger one. Either way you get the scope, price and timeline in writing before we start, and you see progress every week.

What you can customize on each Shopify plan

What you want to change	Every plan (except Starter)	Shopify Plus
Logo, colours, fonts, layout	Checkout editor	Checkout editor, plus the Checkout Branding API for deeper styling
Content, upsells and custom fields	Thank you and Order status pages	Also the information, shipping and payment steps
Discount, payment, delivery and validation logic	Shopify Functions from public apps	Also custom apps with your own Functions
Tracking and analytics	Web pixels	Web pixels

In practice, Plus is where checkout page customization gets serious, but a non-Plus store is far from stuck. We start by showing you what your plan already allows. If you are weighing the upgrade, our Shopify Plus agency page explains how we plan it.

Checkout blocks without custom code

Not every checkout needs a custom build. For common checkout blocks such as trust badges, notices, upsells, custom fields and shipping progress bars, a no-code app is the faster route. We build one: Checkout Ninja’s no-code checkout blocks let your team add ready-made blocks to a Shopify Plus checkout without writing code. When an app covers it, we will say so. Custom extensions are for the rules and experiences an app cannot express.

The two we get asked for most

A countdown or reservation timer inside the checkout, built so it survives the step changes instead of restarting, and an opt-in shipping protection or warranty offer that adds margin to every order that takes it. Both are checkout UI extension work, and both have their own page.

Want the full background first?

For a plain-English explainer of how checkout extensibility works, every extension target and what each plan can do, read the Shopify checkout extensibility guide on Checkout Guy. For the Scripts side specifically, see what replaced Shopify Scripts and our Scripts to Functions migration service.

Start with an audit

Tell us what your checkout used to do, or what you want it to do next. We will map it to the supported stack and come back with a fixed scope, price and timeline. Talk to the team about your checkout.