If you've been putting off the move to Checkout Extensibility, the deadline is approaching. Shopify is retiring the old checkout.liquid approach, and honestly, the new system is better once you understand what it can do.

What Changed

The old way: edit checkout.liquid directly, hope Shopify doesn't break your customisations with an update. The new way: build extensions using a defined API that Shopify maintains and upgrades.

It's more structured, which means less flexibility in some ways but more stability and better performance overall.

What You Can Actually Build

UI Extensions

Add custom elements to the checkout at predefined points—product information, shipping options, payment selection, order summary, and thank you page.

We've built things like:

Delivery date pickers that sync with warehouse schedules
Gift message fields with character limits and formatting
Loyalty point displays and redemption interfaces
Trust badges and security reassurances at the payment step
Upsell and cross-sell widgets based on cart contents

Shopify Functions

This is where it gets interesting. Functions let you run custom logic that affects how the checkout behaves:

Function Types:

local_offerDiscount Functions: Custom discount logic beyond what Shopify's built-in discounts can handle.
local_shippingDelivery Customisation: Hide, rename, or reorder shipping options based on cart contents or customer data.
credit_cardPayment Customisation: Show or hide payment methods based on order value, customer type, or location.
shopping_cartCart Transform: Automatically add, remove, or modify cart items (great for bundles and gifts-with-purchase).

What You Can't Do

There are limitations. You can't completely redesign the checkout layout. You can't inject arbitrary JavaScript. The styling options are more constrained than they were with checkout.liquid.

For most stores, this isn't a problem. For brands with very specific checkout requirements, it might mean compromising on some features or getting creative with the available options.

Getting Started

If you're on Shopify Plus and haven't migrated yet, now's the time. The core functionality is stable, the documentation is solid, and waiting just means more pressure when the deadline hits.