Checkout extensibility, one year after checkout.liquid
What you can and cannot customise now that checkout.liquid is gone, and where the remaining gaps are.
What you get back
Checkout UI extensions cover most of what merchants used checkout.liquid for: custom fields, trust badges, gift messages, delivery instructions, upsells at the thank-you page, and conditional content by cart contents or customer tag. They render in defined slots, they are sandboxed, and they survive Shopify's checkout updates — which is the actual trade.
What you do not
You cannot restructure the checkout layout, change the step order, or inject arbitrary CSS across the page. Branding is handled through the checkout branding settings, not stylesheets. If your old checkout had a heavily custom two-column layout, plan for it to look like Shopify's checkout with your colours and logo, and budget the conversation with your brand team early.
Functions are where the logic lives now
Discount stacking rules, volume pricing, shipping method filtering and payment method hiding are Shopify Functions, written in Rust or JavaScript and deployed as part of an app. They run server-side, so they cannot be bypassed from the client — a genuine improvement over the script-based approach they replaced.
Migration order that works
Inventory what the old checkout did, split it into UI (extensions), logic (Functions) and branding (settings), then build logic first. Logic bugs cost money; a missing trust badge does not. Test with a draft order on a duplicate theme before touching the live checkout profile.
The B2B detail people miss
B2B checkouts have their own profile and their own extension targets. An extension published to your DTC checkout will not appear for company-account buyers unless you deploy it there too. Check both, on a real company account, before you call the migration done.
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