Shopify Checkout Extensions

Overview

Shopify Checkout Extensions allow apps to render custom UI blocks inside Shopify's checkout without forking the checkout template. Shopify Functions let you replace backend logic — discounts, shipping, payment methods, and order validation — with custom WebAssembly modules that run inside Shopify's infrastructure. Together they replace the deprecated checkout.liquid customization approach and work with both Shopify Plus and non-Plus stores (UI extensions) or Plus-only (some Functions targets).

When to Use This Skill

• When adding custom UI blocks to checkout (upsells, trust badges, gift message fields, warranty options)
• When implementing custom discount logic beyond native Shopify discount rules (e.g., tiered discounts, B2B pricing)
• When creating custom shipping method filtering or renaming based on cart contents
• When building payment customization to hide/rename payment methods for specific customers
• When validating order contents before checkout completes (e.g., quantity limits, region restrictions)
• When replacing deprecated checkout.liquid customizations for Shopify Plus stores

Core Instructions

1. Scaffold an extension with Shopify CLI

   # Inside an existing Shopify app directory
   shopify app generate extension
   # Choose: Checkout UI extension OR Shopify Function
   # Name: my-checkout-extension
   

   This creates an extensions/my-checkout-extension/ directory with src/index.tsx (UI) or src/index.ts (Function).

2. Build a Checkout UI extension

   Checkout UI extensions use a React-like component API from @shopify/ui-extensions-react/checkout:

   // extensions/order-upsell/src/index.tsx
   import {
     reactExtension,
     useCartLines,
     useApplyCartLinesChange,
     useSettings,
     BlockStack,
     Button,
     Image,
     Text,
     InlineStack,
     Divider,
   } from "@shopify/ui-extensions-react/checkout";
   
   const CheckoutBlock = reactExtension(
     "purchase.checkout.block.render",
     () => <OrderUpsell />
   );
   export { CheckoutBlock };
   
   function OrderUpsell() {
     const cartLines = useCartLines();
     const applyCartLinesChange = useApplyCartLinesChange();
     const { upsell_variant_id: upsellVariantId } = useSettings();
   
     // Only show upsell if a variant is configured and cart doesn't already contain it
     if (!upsellVariantId) return null;
   
     const alreadyInCart = cartLines.some(
       (line) => line.merchandise.id === upsellVariantId
     );
   
     if (alreadyInCart) return null;
   
     const handleAddUpsell = async () => {
       await applyCartLinesChange({
         type: "addCartLine",
         merchandiseId: upsellVariantId,
         quantity: 1,
       });
     };
   
     return (
       <BlockStack spacing="base">
         <Divider />
         <InlineStack blockAlignment="center" spacing="base">
           <Image source="https://cdn.shopify.com/s/files/..." aspectRatio={1} />
           <BlockStack>
             <Text emphasis="bold">Add a gift bag for $3.99</Text>
             <Text appearance="subdued">Beautiful packaging for your order</Text>
           </BlockStack>
           <Button onPress={handleAddUpsell}>Add</Button>
         </InlineStack>
       </BlockStack>
     );
   }
   

3. Configure the extension in shopify.extension.toml

   api_version = "2025-01"
   
   [[extensions]]
   type = "ui_extension"
   name = "Order Upsell"
   handle = "order-upsell"
   
   [[extensions.targeting]]
   module = "./src/index.tsx"
   target = "purchase.checkout.block.render"
   
   [extensions.settings]
   [[extensions.settings.fields]]
   key = "upsell_variant_id"
   type = "variant_reference"
   name = "Upsell Product Variant"
   

4. Build a Shopify Function for custom discounts

   Shopify Functions compile to WebAssembly. Use Rust or JavaScript:

   // extensions/volume-discount/src/index.ts
   import type {
     RunInput,
     FunctionRunResult,
     CartLineInput,
   } from "../generated/api";
   
   const NO_CHANGES: FunctionRunResult = { discounts: [], discountApplicationStrategy: "FIRST" };
   
   export function run(input: RunInput): FunctionRunResult {
     const { cart } = input;
   
     // Calculate total quantity across all lines
     const totalQuantity = cart.lines.reduce(
       (sum, line) => sum + line.quantity,
       0
     );
   
     // Tiered volume discount
     let discountPercent = 0;
     if (totalQuantity >= 20) discountPercent = 20;
     else if (totalQuantity >= 10) discountPercent = 10;
     else if (totalQuantity >= 5) discountPercent = 5;
   
     if (discountPercent === 0) return NO_CHANGES;
   
     return {
       discounts: [
         {
           value: {
             percentage: { value: discountPercent.toString() },
           },
           targets: [{ orderSubtotal: { excludedVariantIds: [] } }],
           message: `${discountPercent}% volume discount (${totalQuantity} items)`,
         },
       ],
       discountApplicationStrategy: "FIRST",
     };
   }
   

   Function shopify.extension.toml:

   api_version = "2025-01"
   
   [[extensions]]
   type = "function"
   name = "Volume Discount"
   handle = "volume-discount"
   runtime = "javascript"
   
   [[extensions.input.variables]]
   name = "cart"
   type = "Cart"
   
   [extensions.build]
   command = "npm run build"
   path = "dist/index.wasm"
   

5. Test and deploy extensions

   # Run local dev preview (UI extension hot-reloads in checkout)
   shopify app dev
   # Open the checkout preview URL shown in terminal
   
   # Deploy all extensions to Shopify
   shopify app deploy
   

   After deploying, go to Admin → Checkout → Customize to add the UI extension block to a checkout template. Functions are activated by creating a discount with the function from Admin → Discounts.

Examples

Gift message field using useApplyMetafieldsChange

import {
  reactExtension,
  TextField,
  useApplyMetafieldsChange,
  useMetafield,
  BlockStack,
  Text,
} from "@shopify/ui-extensions-react/checkout";

export default reactExtension(
  "purchase.checkout.shipping-option-list.render-after",
  () => <GiftMessage />
);

function GiftMessage() {
  const giftMessage = useMetafield({ namespace: "custom", key: "gift_message" });
  const applyMetafieldsChange = useApplyMetafieldsChange();

  return (
    <BlockStack spacing="tight">
      <Text emphasis="bold">Gift message (optional)</Text>
      <TextField
        label="Message"
        value={giftMessage?.value ?? ""}
        multiline={3}
        onChange={(value) =>
          applyMetafieldsChange({
            type: "updateMetafield",
            namespace: "custom",
            key: "gift_message",
            valueType: "string",
            value,
          })
        }
      />
    </BlockStack>
  );
}

Payment customization Function (hide cash on delivery for international orders)

// extensions/payment-customization/src/index.ts
import type { RunInput, FunctionRunResult } from "../generated/api";

export function run(input: RunInput): FunctionRunResult {
  const country = input.cart.buyerIdentity?.countryCode;

  // Hide "Cash on Delivery" for non-domestic orders
  const hideOperations = input.paymentMethods
    .filter((pm) => pm.name.toLowerCase().includes("cash on delivery") && country !== "US")
    .map((pm) => ({
      hide: { paymentMethodId: pm.id },
    }));

  return { operations: hideOperations };
}

Best Practices

• Use the purchase.checkout.block.render target for maximum placement flexibility — merchants can drag the block anywhere in the checkout editor
• Keep Function execution under 5ms — Shopify enforces a strict execution time limit; avoid network calls inside Functions (use metafields or Function input variables for configuration)
• Use useSettings() hook to read merchant-configured values from the extension settings schema — avoids hardcoded IDs in extension code
• Never read DOM or use browser APIs in UI extensions — they run in a sandboxed Worker environment without DOM access
• Use @shopify/ui-extensions-react/checkout components only — native HTML and other UI libraries are not available in the extension sandbox
• Test payment and shipping Functions with real checkout sessions — the local dev preview only works for UI extensions; Functions need to be deployed to test
• Version-pin your extension API version — increment the api_version in shopify.extension.toml to access new APIs while keeping backward compatibility
• Handle async operations with loading states — useApplyCartLinesChange is async; show a spinner while the mutation is in flight

Common Pitfalls

Problem	Solution
Extension not appearing in checkout editor	Ensure the extension is deployed (shopify app deploy) and the correct checkout template is selected in Admin → Checkout
Function returns FUNCTION_EXECUTION_TIMEOUT	Move configuration out of runtime logic into Function input metafields; avoid complex loops on large catalogs
useCartLines returns stale data after cart update	Use the returned promise from applyCartLinesChange to wait for checkout to re-evaluate before reading cart lines again
Extension crashes with "Cannot use browser APIs"	Remove document, window, localStorage references — the extension runs in a Worker sandbox
Discount Function not applying	Verify the Function-based discount is active in Admin → Discounts and the customer qualifies per any eligibility rules
Checkout UI extension settings not saving	Settings fields require handle values that match the keys referenced by useSettings() in the extension code

Related Skills

• @shopify-app-development
• @shopify-storefront-api
• @shopify-metafields
• @checkout-flow-optimization
• @shopify-admin-api