Build a Shopify cartTransform Function using the update operation to override the price of a cart line item for custom pricing logic

In-depth guide

Steps

1. Scaffold a cartTransform Function and ensure the extension TOML declares the `cart.transform` target; the update operation requires a Shopify Plus merchant or development store
2. In `run.graphql`, query the cart lines including merchandise IDs, current prices, quantities, and any metafields that drive your pricing rules
3. In your Function run handler, return an `UpdateOperation` for each line you want to reprice, specifying the `price` field with the new `Money` value (amount and currencyCode)
4. You can also override `title` and `image` on the updated line to reflect a custom product presentation alongside the price change
5. Return a no-op (empty operations array) for lines that do not meet the pricing condition to leave them unchanged
6. Deploy and test: confirm the overridden price appears in cart and that checkout total reflects the Function-adjusted amount