Google published a new GMC Help page for UCP (Universal Commerce Protocol) - it's only for eligible merchants/products in the USA at this time & there are links to eligibility requirements & a form to express interest if you want to be an early participant.

"You may see this checkout experience soon on specific surfaces such as AI Mode in Search and Gemini."

In a nutshell:

• There'll be a checkout button on product listings so the transaction happens on Google surfaces, while merchants remain the official sellers.
• Payment processors must support Google Pay, shoppers will use payment methods saved in Google Wallet
• GMC will continue to be the central hub for product data for both organic/paid listings