I jumpseated on United to Buenos Aires a little over a year ago. I flew MIA-EZE, and EZE-IAD. Everybody that I dealt with was very cool.
I did get knocked for taxes out of EZE, but not out of the US. I think it was around $50 or so. It could have been a little less though, I don't quite remember. And by the way, that was an Argentinian thing....not United.
The crew going down was super cool. In EZE, they let me ride in the crew bus downtown from the airport. I threw the driver an extra couple bucks, and he took me right to my hotel.
Have fun. You'll love BA. GORGEOUS women! And be sure to have a steak.