For folks traveling to Nigeria with Extension letter, please go ahead, i assure you there is nothing to panic about, i used Turkish airline . While trying to check in I purposely gave them only my expired green card and the guy asked “do you have an extension letter” i smiled and gave it him, i was checked in within 5 minutes. Everyone at Istanbul airport knows about the extension letter, my port of entry at IAD was the easiest. The guy just said welcome. Hopefully this experience of mine is helpful to folks who are scared