Susie, isn't Girl Math great?
Lisa, here's what happens. If you are out of the country for 8 days (as my husband and his son will be) the exemption is $800 per person. So it wouldn't be a problem.
Another way to avoid duty (shhhh...I did not say this...) is to...um...fudge. Like, by wearing said item. (I wouldn't do this with this dress, but I do it with bras and similar items.) If the tags are off and you are wearing the item, nobody is going to question you. Honestly, they are after bigger fish than that $100 sweater (or whatever).
Also, you do get $200 free if you are gone for 24 hours. So if you can make a number of short trips, taking only what is worth about $200 each time, it's pretty straightforward. I live quite close to the border and have done that.
Another trick -- use a Nordstrom note to pay -- it brings the price down by the note's value.
Still another way is to have a friend from the US mail an item to you from their address as a gift. Sometimes you do get dinged for duty -- they can open packages at will and charge you if they see the tags. But often they do not bother -- it just depends. If the declared value is $40 Canadian, then it's okay.
Of course you are taking a risk in that case. The item might never get to you and then you're out of luck.
I wouldn't use the shadier methods very often. Mostly I declare what I have and pay. Or try to arrange my shopping "binges" for when I or family members will be gone long enough for the larger exemption, as now.