I am AAdvantage Gold and have found that the AA iPhone app will allow me to select Main Cabin Extra (MCE) seats for my upcoming international Saver award ticket at no charge. This is odd since Gold normally needs to pay 50 percent of the normal fee for MCE outside 24 hours departure time. AA.com still does not allow me to select MCE but it recognizes my seat selections from the app. I'm not sure if you have elite status with US but either way, perhaps it is worth a shot to access the itinerary via the app to see if you have a similar experience.