What is the most reasonable way to send a poster to the U.S.

[ypdc_dennis]

Depends on the size and weight of the poster, and whether it is being shipped flat or rolled.

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[reallynicestamps]

Canada Post in a tube for most inexpensive posters. There is a small premium (about $1.50) for a tube.

If the poster is very large, vintage or valuable, you may prefer to use a courier. The big problem Canadian buyers have with UPS et al is that they must charge Canadian import fees on items valued over $20CDN (about $16USD).

This is not a problem for selling into the USA, since Americans have a $800USD (~$1000CDN) duty free allowance.