Upon return to US still pick a Citizen/ Green Card line but go straight to assistant there and tell them you have an extension letter. You'll be redirected to a special line for additional checking, which is much shorter then the super long line for US Visitors. And don't even bother checking in at the kiosk, its a total waist of time, it will read your green card as expired.
Also make sure to fill up US Immigration Form that they give you on the plane.
Just got back from Russia, it took me 20 min to go through US custom this way. Last time I didn't pick the right lane and didn't have that Form filled.. ugh.. lost so much time and position in line.. took me over 1h to go through..
Nat26 got a reaction from Frips in Traveling on Extension Letter?