post
https://apis.kentro.live/store/v1/carriers.save
Adds a carrier account (no id; an optional createId of your own makes the add idempotent) or changes one (id). carrier is an option id from carriers.book; expectedVersion is the book's version.
Who may call it: A signed-in buyer (X-Storefront-Buyer); without one the answer is 401 UNAUTHORIZED.
