How do I migrate from Shopify Scripts to Functions?

Move in phases. Run both side by side until parity is proven, then retire Scripts.

How do I add a custom field to the Shopify Plus checkout?

Build a Checkout UI extension that renders the field, validates it client-side, and stores the value either as a cart attribute or against the customer profile via the official APIs — never with brittle DOM injection.

How do I hide a payment method based on cart contents?

Use a payment customisation Function that reads the cart, evaluates your rule (SKU present, total above a threshold, customer tag), and hides or reorders payment methods accordingly.

How do I add an age gate to the checkout?

Render an age-confirmation UI extension at the contact step, validate the answer, and block checkout progression if the customer fails — server-side validation in a Function provides the second line of defence.

Do I need Shopify Plus for Checkout Extensibility?

Yes. Checkout UI extensions and most payment / delivery customisations are Plus-only features.

Is checkout.liquid still supported?

Shopify has announced deprecation of checkout.liquid for Plus stores. New customisation work should be built with Checkout Extensibility.

Can we still use third-party checkout apps?

Yes, when they're built on Checkout Extensibility. We audit the app marketplace alongside custom Function options and recommend the lower-risk path.

Will custom checkout code hurt conversion?

Done right, no. Functions run server-side and UI extensions are React components shipped through Shopify's own pipeline — they're typically faster than legacy script-tag apps.

How long does a checkout customisation project take?

Most engagements are 3–8 weeks depending on the number of Functions, UI extensions, and the complexity of the validation logic.

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Service · Checkout

Checkout Extensibility and Scripts

Tailor every step of the Shopify Plus checkout — pricing, discounts, payments, shipping, validation — with Checkout Extensibility, Functions, and UI extensions.

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Floating translucent checkout panel representing custom Shopify Plus checkout extensibility

What it is

About this Shopify Plus service

Checkout Extensibility is the modern, upgrade-safe way to customise the Shopify Plus checkout. It replaces legacy checkout.liquid with a combination of Checkout UI extensions (React-based components) and Shopify Functions (server-side logic) that survive every platform update.

Most enterprise brands need more than the default checkout. They need B2B-tier pricing, complex discount stacking, custom payment routing, age or identity verification, shipping logic by SKU, and surveys or upsells at the right moment. Done with Checkout Extensibility, all of this is PCI-safe, performant, and maintainable.

Mejix engineers custom checkout flows with Functions for delivery customisation, payment customisation, discounts, and cart transforms — plus UI extensions for branded fields, banners, and informational blocks. Older Plus stores still using Scripts get a clean migration path to Functions, with parity testing on both sides.

What's included

Concrete deliverables

Every engagement ships a defined scope. No vague promises — these are the artifacts and outcomes you can expect.

Discount & cart Functions

Custom discount logic, automatic stacking rules, and cart-level transforms running server-side as Shopify Functions.

Payment customisation

Reorder, rename, hide, or condition payment methods based on cart contents, customer tags, market, or order total.

Delivery customisation

Conditional shipping rules by SKU, weight, destination, or customer segment — without bolt-on third-party apps.

Checkout UI extensions

Branded fields, banners, upsells, and informational blocks placed at the right step of the React-based checkout.

Validation & compliance

Age gates, identity checks, regulated-product blocks, and PCI-safe custom fields written through the official Customer Account API.

Scripts → Functions migration

Audit legacy Shopify Scripts, translate them to Functions, and validate parity before the platform deprecates the old runtime.

How it works

A predictable delivery process

The same sequence we run on every engagement, refined across enterprise Shopify Plus projects.

01

Audit & opportunity map

Inventory current checkout customisations, app dependencies, and pain points. Surface the highest-leverage changes first.
02

Design checkout logic

Write the rules in plain English, then translate them into a Functions + UI extensions architecture with explicit fall-backs.
03

Build, test, parity-check

Implement against a Shopify Plus dev store with synthetic carts covering every branch, including B2B and international.
04

Ship & monitor

Roll out to the live checkout, watch conversion and error rates, iterate quickly on anything that moves.

Why Mejix

A certified Plus Tier partner

We don't sell what we haven't shipped. Every credential below is verifiable on the Shopify Partner Directory.

Plus Tier partner

Checkout Extensibility access is gated by the Plus tier — we work in it every week.

Custom gateway shipped

Real-world experience integrating a custom payment gateway through Checkout Extensibility on a 50,000-SKU store.

Scripts migration ready

We've moved live Plus stores off Scripts and onto Functions without checkout downtime.

How To

How to: Checkout Extensibility and Scripts

Practical answers to the most common 'how do I…' questions teams ask before starting.

Frequently Asked Questions

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Related services & resources

Useful next stops — sibling services on this site and verified pages on mejix.com.