Hi Colin,

We have been mailing packages every year from Europe
for the past 18 years, sometimes 4 - 6 packages, and we
mark them as "gifts". We have never had to pay U.S. duty
or taxes. We keep the value listed below $400 per package,
and have had no problems.
Just look on line at Canadian Customs Postal Regulations
on goods shipped to the USA ....

https://www.canadapost.ca/tools/pg/manual/PGcustoms-e.asp

Al Varelas / USA
SS-1654