To be honest, it's a crapshoot as to whether or not you will get nailed with duty or not. I've ordered all kinds of things from the US and somtimes I've had to pay duty and others times not.. and on exactly the same item! Why it is like this I have no idea. It almost seems like it depends on the person working that day.
If you get hit with duty, you might even end up paying more by the time you factor in the extra shipping and exchange rate as well. Like I said, it's a crapshoot.
I always try to order from Canada whenever possible to avoid the hassle. If you are ordering a bunch of stuff, maybe you could try contacting the guys at ASC Armoury and see if they could cut you some sort of deal. No harm in asking.