Can you advise on how to find previous discussions? I'm having a hard time. Always too many search results.
TimT posted how to do that, too.
Short answer is book through Chase portal a cheaper flight and change flight to the flight that's a few dollars more on United's site in order to use the MP card for the difference and get free luggage.
Or show the card at the airport or try to beg them afterwards to refund it (no guarantees).