Originally Posted by krazykanuck

This. Also, I've found that paying for something associated with that reservation using a Mileageplus card can trigger the free bag waiver (upgrade, E+, etc.) Last summer I had a UA reservation that was booked by our corporate TA on their credit card and it showed I had to pay $25 for the checked bag (didn't have UA status at the time). I bought a cheap upgrade with my MP card and my reservation updated with the waived checked bag fee. YMMV of course.