I didn't know that about the WA State Sales Tax. Interesting if I ever go on a cross-border shopping trip to Spokane. Here's the official info on it:
http://dor.wa.gov/Content/GetAFormOr...dentSales.aspx
It's only for the purchase of goods that will be used outside the state (food, gas, hotel rooms, etc don't qualify), and only for residents of states or provinces with a local sales tax below 3% (I'm not sure why the eastern provinces are eligible). For that reason, Alberta and Montana residents qualify, but BC doesn't.