I'm a Canadian Nexus holder.
I'll be flying back from AMS to YYZ via IAD.
I'd like to bring back my Canadian exempt amount.
But since I'm transferring at IAD for 2 hours, what rules apply?
(In this direction, I'm required to retrieve my checked bag in IAD, pass through US Customs and re-check onward to YYZ).

US alcohol duty is close enough to zero that if you can carry it, generally no duty will be charged. Declare it in the US, and they'll just wave you through.

Because there's no such thing as "international transit" in the United States, you'll be held to US alcohol limits. Though as mnbp mentions, it's rare to be assessed a duty for reasonable amounts, in part because it's a huge hassle for Customs agents.

I don't think this is correct. The customs declaration form instructs visitors to "Declare the value of all articles that will remain in the United States." Visitors aren't required to declare articles that will leave the United States, since that would basically be the entire contents of one's suitcase! (In other words, for visitors, it makes no difference if the goods were brought out from home in Canada to Europe and then back home through the US, or acquired abroad, if the goods aren't staying in the United States.

https://www.cbp.gov/sites/default/fi...tion-6059b.jpg

Yes, of course, declare everything. No money will actually be charged. The amount of alcohol that could fit in a suitcase is unlikely to cost more than a couple of dollars duty - which is why they typically don't bother collecting any duty on personal alcohol. To give an idea, duty on a bottle of wine is something like 10-15 cents. Duty is charged on the alcohol content, not on the value of the wine as an example.

You're suggesting that the previous poster provide false information in the answer to the instruction "Visitors: Declare all goods that will remain in the United States." That would probably raise more questions for the CBP officer: (What will you be doing with those bottles of wine/spirits/whatever during your 2 hour layover? Drinking them? Giving them away? To whom?)

That instruction on the form is different from the instructions to US Citizens, which is "Declare all items acquired abroad."

Actually, some airports *are* set up for ITI/TWOV, at least to a certain extent. For example, DFW Terminal D has a massive (and deserted) international transit lounge located near the passport control checkpoint. I've never seen anyone inside. I suppose it could be used if CBP ever decided to allow ITI/TWOV again. (or maybe it is used infrequently to accommodate inadmissible visitors who are waiting to return to their country of origin)