Originally Posted by
jjmoore
The offer comes up on mobile devices as well. It won't pop up in front of you automatically, but if you go to travel options, you will see it. If a seat is not open, but becomes available closer to departure, it will sow in travel options. Just keep checking.
The one point I would add is there is absolutely no guaranty UA will offer a buy-up on any particular flight, especially TATL or TPAC, regardless of the number of empty seats.