Add one line to a cart

Name carbonguru_id (an item this account analysed, optionally with its session_id) or carbon_kg with a name (an ad-hoc footprint). A session_id on its own names no footprint and is refused, which is what the shipped route does. Adding charges nothing; subscription_mode monthly makes the line recurring and it still charges nothing until a person completes the checkout in a browser. The answer is the named cart, read back.