Originally Posted by
jsloan
Assuming travel is on United -- you can't upgrade with cash ahead of time, and United doesn't have a bidding system. However, you may be offered a cash upgrade at check-in if there are seats available. (The price can fluctuate, so if you don't like the first price that's offered, you can check again later).
Thanks for the quick response. I am surprised that UA would not take money (even if hefty price) ahead of time, but so be it!