Custom Checkout and Extensions
Conversion-focused checkout customisations built on Shopify Checkout Extensibility.
· Reviewed by senior engineers
01 What it is
What this service is
Custom checkout development on Shopify means working within Shopify Checkout Extensibility — the modern, React-based framework that replaced checkout.liquid and the old Script Editor. You build UI extensions that render inside Shopify's hosted checkout, write Shopify Functions for discount, shipping, payment, and delivery customisation in JavaScript or Rust, and customise the post-purchase and thank-you experience.
The big advantage of Extensibility over the old checkout.liquid approach is that you stay on Shopify's upgrade path. Every platform improvement — accessibility fixes, new payment methods, performance work, Shop Pay updates — flows through to your customers automatically. You no longer have to choose between customisation and platform maturity.
The constraint is that you work within the boundaries Shopify defines. Extensions live in approved surfaces, must pass review, and run inside a sandbox. For ninety-five percent of what brands actually need this is fine. For the remaining five percent we will help you decide whether a workaround, a Function, or a different architecture is the right answer.
02 What it's for
What it's for
Checkout customisation is one of the highest-leverage investments in a Shopify brand. Every percentage point of checkout conversion translates directly to revenue. Common engagements: a US apparel brand adding a post-purchase upsell flow that lifts AOV by twelve percent; a UK food brand customising the delivery date picker with Function-based business rules; a German B2B retailer adding a VAT-ID validation step and customer-type-specific payment methods at checkout.
It is also the right call when you need region-specific behaviour. EU customers need clear PSD2-compliant card handling and Klarna or SEPA as primary payment options; LATAM customers need PIX, OXXO, and instalment plans; APAC needs Atome, GrabPay, and local credit card networks. We build the routing logic so the right options surface for the right market without checkout becoming a mess of universal toggles.
And it is the right call for any brand running structured CRO. Custom checkout enables experimentation at the part of the funnel where lifts matter most. We pair the engineering with an experimentation programme rather than shipping one-off changes blind.
03 How to use it
How to engage devinsta
We start with a checkout audit — heat-maps if you have them, GA4 funnel analysis, payment-method drop-off by region, mobile vs desktop friction. The output is a prioritised list of hypotheses with effort and expected lift estimated, plus a roadmap that respects Shopify's review timelines.
Build runs in short sprints because checkout work pays back fast. Each extension is built, tested in a development store, reviewed by Shopify if it is a custom app, and rolled out behind a feature flag so we can A/B test against the unmodified checkout. We never ship a checkout change blind; everything goes through measurement.
Post-launch we typically stay on as a checkout-and-CRO retainer, shipping new tests every two weeks and reviewing aggregate impact monthly. The platform itself moves fast — new extension surfaces, new Functions APIs — so part of the value is keeping your checkout on the front edge of what Shopify supports.
04 How to deploy
How we deploy it
Extensions are deployed via the Shopify CLI from your Git repository. Each extension is a small TypeScript or React package with its own toml manifest; CI/CD runs lint, type-check, and a preview deploy to a development store on every PR. Production deploys go through tagged releases with a clear rollback to a previous extension version.
Shopify Functions run on Shopify's infrastructure — there is nothing to deploy on the hosting side beyond pushing the compiled WASM. We write Functions in TypeScript or Rust depending on the complexity. For Functions that pull configuration from external systems (custom shipping rules driven by an external ERP, for example) we cache the config in metafields with a scheduled sync job rather than calling out at runtime, because Functions have strict execution budgets.
Observability for checkout extensions is the part most teams skip. We instrument every extension with its own event stream into your analytics warehouse — render counts, interaction counts, error rates per surface — and surface them in a dashboard with conversion overlay so you can correlate extension behaviour with revenue. PSD2, PCI DSS, and GDPR posture stay with Shopify since the checkout itself remains Shopify-hosted, and we document any data we capture in extensions for your privacy team.
05 What we provide
What you get from us
- Checkout funnel audit with prioritised hypotheses and expected lift estimates
- Custom checkout UI extensions for cart, information, shipping, and payment steps
- Shopify Functions in JavaScript or Rust for discount, shipping, payment, and delivery rules
- Post-purchase upsell flow with A/B-tested offer logic
- Thank-you page customisation, including referral and review-collection components
- Region-aware payment-method routing and B2B-specific checkout behaviour
- A/B testing infrastructure with feature-flagged rollouts and conversion dashboards
- Documentation, code review, and a six-month support window per extension
FAQ
Common questions
Can we still customise checkout if we are not on Shopify Plus?
Checkout Extensibility is a Plus-only feature for most surfaces. Some basic customisation works on regular Shopify, but the meaningful work — custom UI in checkout, post-purchase, Functions for discount and shipping — requires Plus. If you are on Advanced Shopify and serious about checkout work, upgrading to Plus usually pays back fast.
What about the old checkout.liquid customisations we still have?
Shopify is deprecating checkout.liquid. We migrate existing customisations to the new Extensibility model — usually a mix of UI extensions and Functions — and validate that everything still works as expected. Migration is a fixed-scope engagement; most brands complete it in four to eight weeks.
How long does a custom checkout extension take to build?
A focused extension — a post-purchase upsell, a custom delivery date picker, a B2B-specific field — typically takes two to four weeks from brief to live, including A/B test setup and Shopify review where required. Larger programmes spread across multiple extensions and run as a rolling retainer.
Will checkout extensions slow down our conversion?
Only if they are built badly. Extensions render on Shopify's edge infrastructure, are heavily sandboxed, and have strict performance budgets. We profile every extension in development and watch real-user metrics post-launch. If a new extension hurts conversion we either fix it or pull it; that is what the A/B test is for.