Updates an existing bundle transaction in place - when a customer edits their bundle in the cart (changing products, quantities, discount, box size, or selling plan) before purchase.

Previously you had to call Create transaction again for every edit and rewrite the cart line with a new txnId. With this endpoint you keep the same transactionId across edits and simply refresh the cart.

Auth: Storefront-callable (no token)

This API is only applicable before purchase is completed. A transaction that has been purchased - edit those with Update subscription bundle instead.

Example flow

1. Customer adds a product → call Create transaction → gets transactionId.
2. Call /cart/add.js (with _bundleId: transactionId) to open the cart.
3. Customer edits the bundle in the cart.
4. call Update transaction with the same transactionId.
5. Call /cart/add.js again to reflect the updated cart. (The _bundleId already on the lines stays valid.