Northbound.is works so that you cannot book a vehicle when the suppliers offices are closed. So if you book a vehicle for pickup at the Keflavik International Airport at 04:00 in the morning, then the offices are open and an agent will be there.

Usually agents monitor your flight number and wait for you at the arrival hall right after you leave customs, he will stand there holding a sign with either your name on it or their own logo (visible on your voucher).