As to my knowledge you would have to pay $25 for this trip (roundtrip to South America). If you booked through a reservation system, they will ask for your reservation code. Usually they reply within a few hours, but since it is the weekend, it might take until tomorrow if you try it today. The only way you can get a seat in Economy Plus is if you are a Mileage Plus Premier or better OR
you hold a ticket in Y, B or M booking classes.