Shopify checkout development: extensions, Functions and post-purchase pages

Shopify checkout is customized with four tools: Checkout UI extensions for interface, Shopify Functions for logic, branding settings for look and feel, and apps that package these for you. The old approach, editing checkout.liquid and injecting scripts, is gone. These guides cover each tool, what needs Shopify Plus, and when a no-code app is enough instead of custom development.

• Checkout customizationHow to customize the Shopify checkout page in 2026: branding in the checkout editor, custom fields, content and upsells with Checkout UI extensions, discount, delivery and payment logic with Functions, and what each option needs.
• Checkout UI extensionsHow Shopify Checkout UI extensions work: targets, the Preact and Polaris web components API, settings, cart metafields, blocking progress, network access and plan limits, with a complete extension example.
• Checkout Blocks vs customWhen Shopify's Checkout Blocks app or another no-code checkout app is enough, and when you need a custom Checkout UI extension or Function: plan availability, typical use cases, limits and a decision checklist.
• Order status & Thank you pageHow to customize Shopify's Thank you and Order status pages now that additional scripts, checkout.liquid and script tags are gone: web pixels for tracking, checkout and customer account UI extensions for content, with dates and code.

The four tools

What changed, and when

These are the dates, from Shopify's own documentation, behind the migrations many stores still have left:

• checkout.liquid is no longer supported for the information, shipping and payment steps.
• checkout.liquid and additional scripts were sunset on the Thank you and Order status pages on August 28, 2025.
• Script tags stopped running on the Order status page for Plus stores on August 28, 2025, and for all other stores on August 26, 2026.
• Shopify Scripts (Script Editor) could no longer be edited or published after April 15, 2026 and stopped executing on June 30, 2026. Their logic has to move to Shopify Functions.
• On storefronts (not checkout), apps can't create or update script tags from October 1, 2026, and Shopify stops injecting them on March 1, 2027.

If anything in your store still depends on one of these, it has either already stopped working or will soon.

Which guide do you need?

• "I want to change what checkout looks like or asks for." Start with checkout customization, then Checkout UI extensions for the technical detail.
• "I want to change prices, shipping options or payment methods." Read about Shopify Functions.
• "I had Scripts." Read Scripts to Functions migration.
• "I need tracking or content on the Thank you page." Read Order status and Thank you page customization.
• "Can't I just install an app?" Often, yes. See Checkout Blocks vs a custom extension.

Common checkout projects

• Replacing Scripts with Functions: tiered discounts, shipping rules, payment rules.
• Custom fields that reach the order and an integration: delivery dates, PO rules, tax IDs.
• B2B checkout: payment terms, PO validation, credit limits.
• Moving tracking to web pixels and rebuilding Thank you and Order status content as extensions.
• Validation that stops bad orders: restricted items, address rules, quantity limits.

What a checkout project looks like

1. Requirements: what should the buyer see, and what should the order contain? Which markets, customer types and devices?
2. Plan check: which pieces need Plus, and can a public app cover them instead?
3. Build extensions and Functions in one app with Shopify CLI, with merchant-editable settings where it helps.
4. Test in a development store, then in the checkout editor preview on the live store, then roll out. Extensions are placed and removed in the checkout editor, so rollback is quick.
5. Monitor: Functions and extensions have logs in the Dev Dashboard. Watch checkout conversion before and after.

Checkout is where a mistake costs sales, so every change should be small, tested and easy to reverse.

Decision checklist

• Which checkout steps does the change affect? Information, shipping and payment need Plus.
• Is it UI (an extension), logic (a Function) or both?
• Does an existing app already do it well enough?
• Where does the data go: an order metafield, an attribute or an integration?
• How will you know it works: test cases, conversion and support tickets?