Just a heads-up! At least on NW flight you can purchase duty-free and it will be checked through to your final destination in the US (you have to take it through customs at your first US port of entry) at the gate before boarding. I saw this as I flew out of PEK on Dec 24. I am not sure, but this will likely be true for other US airlines out of PEK, and possibly also from other Chinese airports.

Best wishes for the Holiday Season.