What happens if the user taps on the cancel button after the fueling was already started? Would they get a canceling error?

The session cannot be cancelled once it’s started. The user would get an error. The session can only be cancelled after reserving the pump and getting the authorization but before picking up the nozzle. So, in other words, after getting the authorization, the user can either cancel the session or get out of the car and start fueling, for which he has 90 seconds. He can also do nothing for 90 seconds and then the session would be closed automatically.