Build a Shopify cartTransform Function using the merge operation to group multiple compatible cart lines into a single bundle parent presentation

In-depth guide

Steps

1. Scaffold a cartTransform Function at API version 2025-07; the merge operation uses the `linesMerge` operation name in this version
2. In `run.graphql`, query all cart lines with merchandise IDs and any attributes or metafields that identify which lines should be grouped into a bundle
3. In your Function run handler, identify sets of cart lines that form a complete bundle (e.g., a specific combination of SKUs) and return a `MergeOperation` (called `linesMerge` in 2025-07+) for each group
4. Provide the `parentVariantId` of the bundle parent product that should represent the merged group in cart, along with a `price` override if needed
5. Return all non-merged cart lines unchanged in your output to avoid inadvertently removing unrelated items
6. Validate the merge output in a development store: the grouped lines should collapse into a single parent cart line with the correct quantity and price