Send bookings to the database early, with an expiry time and a counter visible on the screen? Thing is, because purchasing is often shared across third-parties

Send bookings to the database early, with an expiry time and a counter visible on the screen? Thing is, because purchasing is often shared across third-parties, even then you can’t guarantee that you’ll have the reservation until it’s paid and entered into the shared booking backend… unless the third party backend has a locking mechanism, at which point you’re probably not using your own Postgres to do this, but their mainframe instead.