It would work just as most shopping carts do I imagine. You could store a session id in a cookie, and then use the data in KV to map that session id to a user a

It would work just as most shopping carts do I imagine. You could store a session id in a cookie, and then use the data in KV to map that session id to a user account (if they're logged in) and their cart. So you would have a namespace full of sessions, and a namespace full of carts. You could also store carts by session id until they're logged in, and then store the carts by account id when they are.