What Is Shopify Checkout Extensibility?
If you're on Shopify Plus, this deadline already affected you: checkout.liquid was deprecated in August 2024. Any brand still relying on the old checkout templating system is now locked out of Shopify's new one-page checkout, post-purchase features, and App Store integrations.
Checkout Extensibility is Shopify's replacement — a structured, app-based system for customising every stage of the purchase funnel without touching Shopify's core codebase.
The result? Faster checkouts, safer upgrades, and — critically — more revenue per visitor.
The Three Pillars of Checkout Extensibility
1. Checkout UI Extensions
These are React-based components that render directly inside the checkout UI. You can add them to specific slots — before the shipping section, after the order summary, inside the payment step — without rewriting checkout HTML.
What brands use them for:
- Custom upsell widgets ("Add insurance for £2.99")
- Loyalty point displays ("You'll earn 320 points with this order")
- Trust badges and security seals
- Subscription toggle (one-time vs. subscribe and save)
- Custom gift message fields
- Post-purchase review or referral prompts
Extensions are sandboxed — they run in an iframe-like environment — so they can't break the checkout even if your app has a bug. Shopify maintains full control over layout and performance.
2. Shopify Functions
Functions run server-side on Shopify's infrastructure. They allow you to override core commerce logic — something that was previously impossible without moving to headless.
You can use Functions to:
- Create custom discount logic (e.g. "10% off if cart contains 3+ products from tagged collection")
- Build tiered pricing for B2B wholesale customers
- Customise shipping rate filtering (hide certain carriers based on postcode)
- Implement custom payment method filtering (hide BNPL for orders under £20)
- Write custom cart validation rules
Functions are written in Rust or AssemblyScript (or via third-party SDKs that abstract this) and execute in under 5ms. They replace the old Script Editor — which will also be deprecated.
3. Checkout Blocks (No-Code)
For merchants without a developer, Shopify introduced Checkout Blocks — a drag-and-drop editor inside the Checkout Editor (equivalent to the Theme Editor but for checkout).
You can use pre-built blocks from the App Store, or have a developer build custom blocks for your brand. This gives your merchandising team control over checkout content without engineering tickets.
Why This Matters for Conversion Rate
The average Shopify store sees a 1.5–3% checkout completion rate improvement after migrating to one-page checkout. Shopify's own data shows the new one-page checkout converts 36% better in some verticals.
Here's the mechanism:
| Old Checkout (3-page) | New One-Page Checkout | |---|---| | 3 page loads, 3 friction points | Single view, instant progress | | Slow on mobile (200–400ms per step) | Fully optimised by Shopify | | Limited tracking (each page = separate hit) | Unified analytics funnel | | No access to new App ecosystem | Full Checkout Extensibility access |
Brands locked on checkout.liquid cannot access the one-page checkout. This is the most urgent migration reason.
The Migration Process: What's Involved
Step 1: Audit Your Current Checkout Customisations
Before anything else, list every customisation in your current checkout.liquid:
- Custom fields (VAT number, delivery instructions, gift wrap)
- Third-party scripts (loyalty apps, address validation)
- CSS overrides (brand colours, typography)
- Upsell widgets
- Analytics tags
Step 2: Map Each Customisation to the New System
| Old Approach | New Equivalent | |---|---| | Custom HTML field in checkout.liquid | Checkout UI Extension (custom field) | | Inline CSS overrides | Checkout Editor branding panel | | Script Editor discount logic | Shopify Functions | | Third-party upsell app (liquid-based) | App rebuilt on UI Extensions API | | Custom shipping rules | Shopify Functions (delivery customisation) |
Most reputable Shopify apps (Okendo, ReCharge, Gorgias, etc.) have already migrated to UI Extensions. Check your app versions.
Step 3: Build and Deploy Extensions
If you have custom logic that no off-the-shelf app covers, you'll need a developer to build a custom extension. The toolchain is:
npm install -g @shopify/cli
shopify app create
shopify app generate extension --type checkout_ui_extension
Each extension is published through the Shopify Partner Dashboard and deployed via a Shopify app. Extensions go through Shopify's review process (typically 1–5 business days).
Step 4: Test in the Checkout Editor
The Checkout Editor gives you a live preview of each extension slot. Test every combination of:
- Guest vs. logged-in customer
- One-time purchase vs. subscription
- Domestic vs. international shipping
- Discount codes applied vs. not applied
Step 5: Go Live and Monitor
After switching, monitor these metrics for 2 weeks:
- Checkout completion rate (% who reach order confirmation)
- Average order value (upsell extensions impact this directly)
- Cart abandonment rate
- Page load time at checkout
Common Mistakes Brands Make During Migration
1. Waiting too long. Checkout.liquid is already deprecated. Every month you delay is a month your checkout lags behind competitors using one-page checkout.
2. Assuming all apps support it. Some smaller apps still use liquid-based checkout injection and won't work after migration. Audit your entire app stack.
3. Skipping the Checkout Editor branding step. After migration, your checkout may lose brand customisation (fonts, colours, logo size). Redo this in the new Checkout Editor under Settings → Checkout → Customise.
4. Not rebuilding analytics. Many brands have custom checkout step events in Google Analytics or Meta Pixel that were fired via liquid scripts. Rebuild these using the Checkout UI Extension useAnalytics hook.
What a Well-Optimised Checkout Extension Stack Looks Like
Here's a typical Shopify Plus setup post-migration for a DTC brand doing £2M+ ARR:
- Trust/urgency block — "Order in the next 2h 14m for same-day dispatch" (custom UI Extension)
- Loyalty display — ReCharge or Yotpo integrated via their UI Extension app
- Upsell widget — A product recommendation or gift wrap add-on via Checkout Blocks
- B2B field — VAT number input for trade customers (custom UI Extension)
- Functions — Volume discount logic for bundles; BNPL filter for orders under £30
- Post-purchase page — Cross-sell, referral incentive, or review request (Thank You Page extension)
Should You Do This Yourself or Hire a Shopify Developer?
DIY is viable if:
- You only need Checkout Editor branding changes (no custom logic)
- Your apps are all already UI Extension-compatible
- You have an in-house developer familiar with React
Hire a Shopify development agency if:
- You have custom checkout fields, discount logic, or shipping rules
- You're building net-new upsell or loyalty features
- Your migration timeline is critical (peak season approaching)
- You want the migration done correctly first time with zero revenue downtime
At Developia, we specialise in Shopify Plus checkout migrations. We audit, plan, build, and deploy — with a checkout performance guarantee built into every project.
The Bottom Line
Checkout Extensibility isn't optional. It's the foundation of every checkout improvement Shopify is building in 2026 and beyond. Brands that migrate now are already compounding the gains: better conversion, higher AOV from native upsells, and access to Shopify's upcoming checkout innovations.
If you're still on checkout.liquid — the clock is running.