In the last episode of , John Levine
said:

But that's overly restrictive because it cuts out non-US connections,
which allow the sealed carry-on.

Remember that most US airports do not have customs facilities. If you
can't find a flight from London or Amsterdam or Frankfurt to a city,
you can be reasonably sure that there aren't any overseas flights to
it at all.

One possibility is to connect at an airport with US preclearance. I
know that in Vancouver, Toronto and Montreal, you go directly from an
incoming international flight to US customs. CATSA says they allow
duty free security bags but it's not clear whether they allow them
on US flights:

http://www.catsa.gc.ca/duty-free-purchases

If they don't, you have a problem, because they have an expedited baggage
system where you don't normally collect your bags when you connect, so you
can't easily stick them in your checked luggage.

If you're passing through customs, you can expect to retrieve your bags
and re-check them. The bags are classified as "checked through", but in
reality, you still touch your bag at one point as customs wants to have
you and your bags together if they decide to inspect you.

(Unless this has changed very recently -- I last did this type of
connection some months ago, but it definitely hasn't changed when you
enter into Canada and have a connecting flight)

--
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