You upgraded to Shopify Plus partly to customise your checkout. Then you discovered that checkout.liquid was deprecated in August 2024 and the old approach no longer works. Now you need to understand what replaced it, what you can actually build, and whether the investment is worth it for your store.

The short answer: Checkout Extensibility is a significant improvement over checkout.liquid. It is more structured, more maintainable, and survives Shopify's own upgrades without breaking. But it requires a different approach to building, and the learning curve for developers unfamiliar with React-based UI Extensions is real.

What was checkout.liquid and why was it deprecated?

checkout.liquid was a theme file that let Plus merchants inject custom HTML, CSS, and JavaScript directly into the checkout. It was powerful but fragile. Every time Shopify updated its checkout, merchants with custom checkout.liquid implementations risked their customisations breaking. Maintaining it required developer intervention on every significant Shopify release.

Shopify deprecated checkout.liquid in August 2024. Stores that had been relying on it were migrated to the new Checkout Extensibility framework. If you are still expecting checkout.liquid to work, it does not. You need to build on the new system.

What is Checkout Extensibility?

Checkout Extensibility is a set of APIs and extension points that let Plus merchants customise the checkout experience in a way that is sandboxed from Shopify's own code. Your customisations live in isolated extension components rather than being injected directly into the checkout HTML.

This means:

Shopify can update its checkout without breaking your extensions
Your extensions are reviewed and approved, reducing the risk of introducing security vulnerabilities
You can use the Checkout editor in your admin to position and configure extensions without deploying code

The framework has three main components: Checkout UI Extensions, Checkout Functions, and the Branding API.

What can you build with Checkout UI Extensions?

Checkout UI Extensions are the visible components you add to the checkout page. They render in specific slots within the checkout layout.

Post-purchase upsells: Show a targeted product recommendation after the customer has placed their order but before they leave the confirmation page. This is one of the highest-ROI applications. Industry benchmarks suggest post-purchase upsells convert at 3-8% of eligible orders when the offer is relevant and priced correctly.

Custom fields: Capture additional information during checkout without redirecting customers elsewhere. Common use cases:

Gift messages
Delivery instructions
VAT number or company name for B2B orders
Engraving or personalisation details
Preferred delivery date selection

Trust badges and social proof: Display security badges, review scores, delivery promise messaging, or specific guarantees within the checkout flow to reduce abandonment at the payment step.

Loyalty point display: Show customers their current loyalty balance and how many points they will earn on the current order. This is particularly effective for customers who are loyalty programme members and may not be aware of their balance during checkout.

Gift message and wrapping options: Offer gift wrapping as an upsell directly in checkout rather than through a product page add-on, which typically sees lower take-up rates.

Order summary customisations: Modify what appears in the order summary section, including adding promotional messaging, delivery date estimates, or custom product information.

What are Checkout Functions?

Checkout Functions operate on the server side and allow you to modify checkout logic, not just the visual presentation. They run as compiled WebAssembly functions within Shopify's infrastructure.

Key applications:

Custom discount logic: Apply complex discount rules that go beyond what Shopify's native discount engine supports. For example, tiered discounts based on cart contents, customer segments, or external loyalty data.
Shipping customisation: Show, hide, or rename shipping options based on cart contents, customer location, or order value. You can also calculate custom shipping rates.
Payment customisation: Show or hide specific payment methods based on order value, customer type, or products in the cart. Useful for hiding instalment payment options on low-value orders or surfacing net-terms options for B2B customers.
Cart validation: Block checkout completion if specific conditions are not met, such as minimum order quantities for certain product combinations.

Shopify Functions are the broader category. Checkout Functions are the subset that specifically modify checkout behaviour.

The Branding API

The Branding API lets you control the visual design of the checkout: colours, typography, border radius, button styles, and logo placement. This is available through the checkout editor in your theme settings rather than through code, which means a non-developer can handle most brand alignment work.

The Branding API covers far more visual control than the old checkout CSS injection approach, and it is forward-compatible with Shopify's checkout updates.

Which apps work with Checkout Extensibility?

Several third-party apps have been built specifically for the Checkout Extensibility framework:

Checkout Blocks

Checkout Blocks is the leading no-code solution for building checkout UI extensions. It provides a drag-and-drop interface for adding upsells, custom fields, trust badges, progress bars, and other elements without writing extension code.

Pricing: Free plan available with basic features. Paid plans start at around $99/month for Plus merchants with access to the full extension library.

Best for: Merchants who want checkout customisations without developer involvement, or who want to prototype and test ideas before committing to custom development.

Rebuy

Rebuy is a personalisation and upsell engine that integrates deeply with Shopify Plus checkout. Its Smart Cart and checkout upsell features use your store's order history and product data to generate personalised recommendations.

Pricing: Plans start at around $99/month, scaling with store revenue.

Best for: Merchants with a substantial catalogue where personalised product recommendations at checkout would be relevant. Particularly effective for consumable or replenishment products.

Checkout Plus (by ShopPad)

A broader checkout customisation app with support for custom fields, upsells, delivery date pickers, and trust elements.

Pricing: Around $99/month for Plus merchants.

Best for: Merchants who want a mid-range option between Checkout Blocks and custom development.

What does custom Checkout Extensibility development cost?

If you need customisations that go beyond what apps like Checkout Blocks offer, you will need a developer with experience in Shopify's extension APIs (React, TypeScript, and the Shopify Checkout UI Extensions SDK).

Typical project costs:

Scope	Estimated Cost
Single custom field addition	£500 to £1,500
Post-purchase upsell component	£1,500 to £3,500
Custom discount function	£2,000 to £5,000
Full checkout redesign with multiple extensions	£8,000 to £20,000+

These are rough benchmarks. The actual cost depends on the complexity of the business logic involved and whether you need ongoing maintenance or a one-time build.

For many merchants, starting with Checkout Blocks at £99/month is the right approach. Custom development makes sense when your requirements exceed what available apps can deliver, or when the business logic is proprietary enough that a general-purpose app cannot handle it.

What is the business case for customising checkout?

The checkout page already has a high-intent audience. These are customers who have decided to buy. Abandonment at this stage is predominantly driven by trust concerns, friction, and missed upsell opportunities.

Conversion improvement: Reducing checkout abandonment by even 1% can have a significant revenue impact. For a store doing £2m/year with a 70% checkout completion rate, lifting that to 71% adds roughly £28,500 in incremental revenue.

AOV lift via upsells: Post-purchase upsells operating at a 5% conversion rate on a £30 average upsell value across 10,000 annual orders generates £15,000 in additional revenue from zero additional traffic.

Data capture: Custom fields for gift messages, personalisation, and B2B information reduce customer service queries and improve fulfilment accuracy, with measurable operational cost savings.

The investment in checkout customisation is most clearly justified when you have:

A checkout abandonment rate significantly above the Shopify average (around 30%)
A high AOV where upsell relevance is strong
A B2B channel requiring specific field capture during checkout
A loyalty programme where point display at checkout will drive repeat purchase intent

Key actions to take now

Audit your current checkout: Use Shopify's analytics to identify where customers are dropping off in the checkout funnel. If abandonment spikes at the payment step, trust badge extensions may have the highest immediate impact.
Start with Checkout Blocks: Install Checkout Blocks on a development store and experiment with the available extension types before committing to custom development. Many merchants find it covers 80% of their requirements.
Identify your highest-value upsell opportunity: Review your order data to find the product or category most commonly purchased alongside your bestsellers. Build your first post-purchase upsell around this pairing.
Scope your custom field requirements: If you have B2B customers checking out on your DTC store, or if you offer personalisation, list every piece of additional information you need to capture at checkout. Checkout UI Extensions can handle most of these cleanly.
Brief a Shopify Plus Partner: If your requirements go beyond what apps can handle, brief a Plus Partner agency on your specific needs. Get a fixed-price quote before committing.
Measure before and after: Set up a baseline measurement of your checkout conversion rate before making any changes so you can accurately calculate the ROI of each extension you add.

Frequently Asked Questions

Is Checkout Extensibility only available on Shopify Plus? Yes. Checkout UI Extensions and Checkout Functions are exclusively available to Shopify Plus merchants. Standard plan merchants cannot customise the checkout beyond the Branding API's visual controls.

Do I need a developer to use Checkout Extensibility? Not necessarily. Apps like Checkout Blocks let you build and deploy checkout extensions without writing code. However, custom business logic, complex discount functions, or deeply personalised experiences will require a developer with React and TypeScript skills.

Will my checkout customisations break when Shopify updates its checkout? No. This was the core problem with checkout.liquid that Checkout Extensibility was designed to solve. Your extensions are sandboxed components that are isolated from Shopify's checkout code and will survive platform updates.

How do I migrate from checkout.liquid to Checkout Extensibility? You cannot directly port checkout.liquid code to the new system. You need to rebuild each customisation as a Checkout UI Extension or Function. Start by listing everything your checkout.liquid was doing, then identify which Extensibility component handles each use case. Most merchants find that a combination of a no-code app and targeted custom development covers the full scope.

Shopify Plus Checkout Extensibility: What You Can Build and How to Do It