Originally Posted by ingjald

Hey everyone,

I have a somewhat unusual itinerary coming up, where I am traveling to Japan, and then fly to Canada via the U.S., and then return to the U.S. by train the next day. I know that the U.S. duty-free limit is ordinarily $800, but that limit only applies for trips out of the country that are longer than 48 hours (otherwise, it's $200).

So I have a few questions:

1. Will I go through U.S. Customs when I'm transiting the U.S. to go to Canada? (I believe the answer is yes here)
Yes. The United States has no provisions for international transiting. You must clear immigration and customs.

2. If so, will I declare everything I'm bringing back from Japan when I'm transiting the U.S.? (I believe this is yes as well)
Yes.

3. When returning to the U.S. from Canada the next day, will I declare everything, or only the stuff I obtained while in Canada? If I need to declare everything again, I assume I wouldn't legally owe duty, since I already legally imported it once free of duty, but how can I prove that?
I believe so, but perhaps take a picture of your prior day's customs declaration. Why not ask during your first pass through Immigration the prior day?