I've never flown out of AEP internationally, but I have flown internationally out of MDZ and EZE. You only pay the 18 USD tax if you're leaving the country. I would imagine that this would be charged at AEP if you're leaving to Rio de Janeiro, as RJ is an international desetination.
I don't know with which airline you are flying, but from my personal experience TAM has included the tax in the fare. I read somewhere else on this forum that GOL is also tacking the tax onto the price of the ticket. I'm not sure about Pluna.
(Edit: According to some
research I've done, all airport taxes were to be paid upon purchase of the ticket starting March 1, 2009.)
A partir del 1° de marzo de 2009 cada pasajero deberá abonar su Tasa de uso de Aerostación Internacional a la línea aérea al momento de la compra del pasaje, de conformidad con la Resolución ORSNA 10/2009.
Los menores de 2 años no pagan esta tasa. Los pasajeros en tránsito están exentos de abonarla.
Boa viagem!
