Checkout Extensibility

Shopify Checkout UI Extension Development

Post-purchase upsells, custom order fields, delivery instructions and trust content — built as extensions inside Shopify's own checkout rather than bolted onto it. checkout.liquid is gone, and this is the supported replacement.

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Shopify checkout UI extension adding custom fields and a post-purchase offer inside Shopify checkout

Why it matters

Shopify owns the checkout. Your job is to work inside it well.

checkout.liquid has been deprecated, and the customisations merchants relied on for years no longer have a home. Checkout extensibility replaces it with a sandboxed extension model: your code runs in a web worker, renders through Shopify's own UI components, and can't touch the DOM. That's a constraint, and it's the right one — it's why the checkout stays PCI compliant, keeps Shop Pay and accelerated wallets working, and doesn't break when Shopify ships an update. Be clear on the plan gating, though. Extensions on the Thank you and Order status pages, and post-purchase offers, are available broadly. Extensions inside the checkout steps themselves — along with branding controls, custom fields at checkout and validation rules — are Shopify Plus features. If you're not on Plus, half the ideas in your brief aren't available at any price.

Sound familiar?

How checkout work usually lands on your desk.

Some of these have a clean answer inside checkout extensibility. Others need Plus, or need a different surface entirely.

  • Your checkout.liquid customisations stopped working and nobody on the team owns the replacement.
  • You want a post-purchase upsell that doesn't touch the order the customer just approved.
  • Your B2B buyers need a PO number at checkout, and the notes box isn't cutting it.
  • You assume checkout extensions let you redesign the checkout layout — they deliberately don't.
  • Delivery instructions arrive in three different places and your warehouse ignores all of them.
  • Customers stall at payment because your returns policy and guarantee live two clicks away.

What's included

What a checkout extension build includes

Scoped to what your plan actually permits, then built to survive every payment method and device.

The right extension target

There are many places an extension can render — beside the delivery options, under the order summary, on the Thank you page, in the order status view. We pick targets by where the customer's question actually occurs, not by where there happens to be space.

Post-purchase offers

An upsell shown after payment is captured but before the confirmation page, so the original order is never put at risk. Honest caveat: post-purchase offers don't render for every payment method, so a share of your orders will simply never see one.

Custom fields and instructions

PO numbers, ABNs, delivery notes, gift messages, cut-off acknowledgements — captured at checkout and written onto the order as attributes or metafields, so your 3PL and support team read them from the order rather than from an email thread.

Validation and rules

On Plus, we can block or gate checkout completion — rejecting PO boxes, enforcing minimums for a market, requiring a field before the customer proceeds. This runs through checkout validation and Shopify Functions, not client-side JavaScript that a determined browser ignores.

Shopify's own components

Extensions render with Shopify's checkout component library, which inherits the merchant's branding automatically and stays accessible. You don't get arbitrary CSS or DOM access. Designs that assume otherwise get rewritten in the first week, so we resolve that before anyone builds.

Measured on completion rate

Anything added to a checkout is a risk to the one funnel step you cannot afford to damage. We instrument checkout completion before launch and watch it after, and we'll recommend removing an extension that earns less than it costs in abandonment.

How it runs

How a checkout extension project runs.

Usually 2–5 weeks per extension. Discovery is short. The time goes into testing across payment methods, markets and devices, which is where these builds fail.

  1. 01

    Confirm what's possible

    We check your plan, your existing checkout apps and the surfaces you're asking about, then tell you which items in the brief are buildable now, which need Plus, and which belong somewhere other than checkout.

  2. 02

    Read the checkout data

    Where customers actually drop, which fields support keeps chasing after the order, and what the current abandonment pattern looks like. Without this baseline you can't tell whether the extension helped or quietly cost you orders.

  3. 03

    Build in the sandbox

    The extension is built against Shopify's component library, wired to order attributes or metafields, and reviewed on a development store with the merchant's real branding applied so nobody is surprised by how it looks.

  4. 04

    Test every payment path

    Cards, wallets, Shop Pay, local methods, express checkout, mobile. Post-purchase behaviour differs across them and so does rendering. Then we watch completion rate for a fortnight after launch, not just the day of.

Plus

For in-checkout steps

Thank you page is broader

Sandboxed

Web worker runtime

No DOM, no custom scripts

Native

PCI compliance

Shopify still owns checkout

Post-purchase

Offers after payment

Original order untouched

Straight answer

The honest scope of what checkout extensions can do.

A good fit if…

  • You're on Shopify Plus with specific checkout requirements no settings toggle can cover.
  • You're carrying checkout.liquid customisations that need a supported home before they break.
  • You want post-purchase offers and accept they won't render for every payment method.
  • You sell B2B or regulated goods and must capture extra fields at the moment of order.

Probably not, if…

  • You're not on Plus and want conditional logic or branding inside the checkout steps.
  • You want a bespoke checkout layout — Shopify owns that surface and will keep owning it.
  • Your idea needs DOM access or custom scripts; the sandbox exists precisely to prevent that.
  • You have no checkout analytics baseline — fix measurement first or you'll never know the result.

Keep exploring

More on Shopify app development

Checkout UI Extensions is one part of how we build Shopify apps. Here's the rest of the picture.

The full service

Shopify App Development

The complete service — custom apps, extensions and Functions, in one engagement.

View the serviceClosely related

Shopify Functions Development

The logic behind the UI — discounts, cart transforms and delivery rules running server-side.

Read the guideClosely related

Customer Account Extensions

What happens after checkout: tracking, returns and subscription self-service in the account.

Read the guide

All guides in this series

Beyond this service

Other things we do

Most stores need two or three of these working together. Book a call and we'll tell you which ones actually move your numbers.

View all services

Frequently asked questions

  • For the checkout steps themselves, yes. Customising the information, shipping and payment steps — plus branding controls, custom fields at checkout and validation rules — is a Shopify Plus capability. Extensions on the Thank you and Order status pages, and post-purchase offers, are available much more broadly. So a non-Plus merchant can still do useful work after payment; they can't rebuild the checkout flow itself. We'll tell you which side of that line your brief sits on before you commit budget.

Something specific in mind for checkout?

Book a 30-minute technical call. We'll tell you what's buildable on your plan, what needs Plus, and what isn't possible at all.

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