The Andes Online office is new and bright, next to the Aero Argentina office on the main street in Puerto Iguassu (if you can find Aero Argentina in Lonely Planet, you can locate Andes Online).
I booked the flight two nights in advance and they still have seats. They open late. I was there at around 8:00pm. In the flight, they serve fancy biscuits. Planes looks pretty new. It looks better than United Airline.
Flight left at 2:50, 25 minutes early than the time on my ticket (it makes paying premium for the taxi fair worthwhile).