I'm traveling in a month to the UK with my spouse and child. We went two years ago with an extension letter and had no issues, just some extra processing at check in and when I came back to the US. I was entering our passenger details on the British Airways website, we flew a different airline before. Passport info is straight forward but then it asks for my green card info. I included my USCIS number as it's written on the card but for expiration it doesn't allow me to add a date in the past, for obvious reasons. With the extension letter it should be straight forward like before when I'm traveling. But for my passenger details in the lead up should I put the expiration as it would be with the extension letter, adding 48 months on? Leave everything blank and do it at check in? Call and find out best way to proceed? Just wondering if anything that went through know of the best way to handle this?