Usually you can’t do online check in for international flights, but there is some sort of thing in the online portals for the airlines where you have to provide your identity information at least 48 hours before any flights to the U.S.? That’s when we used the extended green card date. I think I learned to do it that way because his green card was already expired, and they wouldn’t allow me to put and expired date. I was forced to use the “new” expiration date. I wouldn’t hesitate to put it in the online check in either, personally. But to each their own of course
Hubby doesn’t remember them ever asking to see the letter, but they may have at the check in desk. Those travel days are always a blur!