This is why I use American Express; they generally side with consumers. Your one call to United (who should take responsibility; it's their web site) is due diligence, at which point American Express will reimburse and (perhaps) pursue.

The frustrating thing here is that American Express makes a financial decision whether to pursue, or just reimburse. For example, a Jacksonville car rental was adding an airport fee to Hotwire fully prepaid rentals, coding it when pressed as an upgrade fee. Hotwire sided with the rental agency. One can make a lot of money stealing $8 each from 10,000 people, and I wanted these smug buggers tarred and feathered. All American Express would do for me was give me my $8 back.