I'm going to answer my own question as I received a response from the USPCC today. I'll quote them directly.
"The duty charges are inclusive in the quoted per deck price, so there won’t be any additional charges for these.
Only the taxes will be applicable on the invoice. "
Prices quoted in CAD to a Canada address.
It's possible that there's a factor that makes it easier for them to ship to you without duties. They have a Canadian subsidiary, the International Playing Card Company. At one time in history, USPC used to manufacture their cards in several locations in the US, and through their subsidiary, in Canada. Eventually, the company consolidated all its internally-made decks to a single location, which was Norwood, Ohio, a suburb of Cincinnati, until 2009, when they moved to another Cincinnati suburb: Erlanger, Kentucky. IPCC now exists only as a marketing and distribution arm of USPC in Canada, but it might be enough to qualify them as a Canadian company, thus making their decks free of Customs charges. It's just a hunch, but it's certainly possible.