Implement Google Merchant Center feed supplemental data to override and enrich primary feed attributes without modifying the source feed

Verified steps

1. Create a supplemental feed in Google Merchant Center Merchant API targeting the same target country and language as the primary feed, using the SUPPLEMENTAL feed type
2. Structure the supplemental feed as a tab-separated or XML file containing only the id column (matching the primary feed's item IDs) and the attributes to override such as title, description, or custom_label values
3. Register the supplemental feed file URL or configure a scheduled fetch, ensuring the supplemental feed refresh schedule is aligned with the primary feed update cadence
4. Use supplemental feeds to inject computed attributes like sale_price and sale_price_effective_date derived from a pricing system without requiring access to the primary feed source
5. Validate supplemental feed uploads by checking the feed processing report for item-level errors where the id does not match any primary feed item, which causes the supplemental row to be silently ignored
6. Monitor item disapproval rates after supplemental feed activation to detect cases where overridden attributes introduce policy violations not present in the primary feed