You need to call LAN directly 1 866 435 9526 option "2" to add the SA Airpass to you reservation. They are not charging extra to reserve over the phone.
Still the best deal for internal flights in SA.
Welcome to Cruise Critic! If you'd like to participate on our forums by joining in the conversation, please Register Now! Be sure to visit our FAMOUS Roll Call forums, where you can meet other cruisers sailing with you and share a tour or shore excursion and SAVE MONEY!