Auth: Storefront-callable (no token).

If you skip this step, the purchased line items are not bundled - they land on the subscription as separate, unlinked items.

Which IDs go where

Note:

• Create a transaction each time a customer adds a new bundle to the cart.
• If the customer edits the same bundle before purchase, don't create another one - call Update bundle transaction with the same txnId (This API is only applicable before purchase is completed).

⚠️ Getting quantity right (most common mistake)

• productVariants[].quantity is the quantity of that child variant in the bundle. The total number of items in the box is the sum of these.
• Top-level quantity is the number of bundles and defaults to 1. For a BYOB bundle this should stay 1. Do not put the total item count here - passing e.g. 12 because there are 12 items will corrupt the subscription.

✅ Correct - 12 items expressed as child quantities; bundle quantity stays 1:

{
  "boxSizeId": "01HHD6D5XDTFFA83RSZ8XNEKT9",
  "sellingPlanShopifyId": 1051852993,
  "quantity": 1,
  "productVariants": [
    { "shopifyId": 47875709174063, "quantity": 8 },
    { "shopifyId": 47875709174064, "quantity": 4 }
  ]
}

After you get txnId: attach it to the cart

Add the txnId as the line-item property _bundleId: txnId on the Shopify /cart/add.js call, for every bundle line. The key must be exactly _bundleId.