In my limited experience of one order shipped to my home in Toronto, no. I did not have to pay any duty on my order shipped via USPS First Class International. Granted, it is worth noting that this option is only available for orders less than $60 USD.
If you're ordering anything more expensive than that (i.e. a bag), then it gets shipped via UPS International Expedited, which already pre-pays the brokerage. I don't think there should be duty on it since all goods were manufactured in the USA (yay for NAFTA?), but can't confirm that as I've had my big purchases shipped to the US when down there on business. I'm generally against shipping anything via UPS cross-border since they are the WORST at charging ridiculous brokerage fees, but as this service includes it prepaid, I'm hoping it is less of an issue than with eBay or other online shopping shipments.
Just to be safe, I just made my over $60 order and had it shipped to a good friend in Buffalo. Plus I get to spend the day in Buffalo so no complaints from me. (albeit with no money!)