TL;DR Merchant Center's "Invalid value for category [price]" fires whenever the price attribute Google receives is exactly 0; the only documented exception is mobile devices sold under a contract promotion. On Shopify the zero comes from one of two unrelated causes: an unfilled variant price (a data issue, no code involved) or a theme's JSON-LD snippet reading a variable that can resolve to empty (a code bug that hits every product page). Scanning structured data across 90,000+ Shopify...