Originally Posted by PJV33

I'd like to contribute my experience with this. I have the MPExplorer, linked to my MP account and everything. I had my brother book me 6 one way flights on the same RSVP with his credit card all on UA metal over the course of 5 days since he owed me (should've let him mail me a check, I know I know).
The rsvp showed $25 for my first bag for all the flights, total of $150.

I went into the seating and purchased a $16 E+ seat for the shortest of my flights (BOS-EWR) using my Explorer card.
It took a couple of days, but now my entire itinerary is reflecting the $0 bag fee for the first checked bag.

From what I am seeing, as long as you use the card at some point it will connect the dots and link the card to the itinerary.