Google’s Universal Commerce Protocol can now handle shopping carts, live catalog queries, and loyalty program benefits for AI agent transactions. On March 19, Google  announced three new UCP capabilities  and a simplified onboarding path through Merchant Center, two months after Google and Shopify  unveiled UCP  at the National Retail Federation conference in January 2026. The January launch had a big coalition (Mastercard, Visa, Walmart, Target, Best Buy) but limited functionality. UCP could handle single-item checkout sessions and not much else. The March update closes the gap between UCP’s ambition and UCP’s practical capability. I covered UCP in depth in  Selling to AI: The Complete Guide to Agentic Commerce , where I compared UCP to OpenAI and Stripe’s Agentic Commerce Protocol (ACP) . This article covers what changed in March and what the changes mean for retailers. What Google Added Cart.…