Fortunately I called the transportation provider at 4:30 a.m. to confirm our 5:00 a.m. pickup. They had no record of a reservation from Viator and their shuttle van had already left for its early morning pickups. After one phone disconnection, a long wait on hold with a reminder every 10 seconds saying how valuable I was, and two more phone calls, they arranged to pick me up at my hotel on time. However, it was a nerve-wracking 30 minutes. I don't know (and I don't care) whether it was Viator or the provider's fault, but it was not a good experience even if the end result was a timely arrival at the airport.