Well, here's the thing....let's say you find something you want to buy on Amazon....a PC router for $99. You do a bit of searching on the 'net and find that the deal on Amazon is actually from a dealer out of state that you could order from directly and definitely pay NO TAX, then neither would you pay tax in Washgtn. I believe if Amazon is warehousing the product in question or the prod actually comes from Wash state, then you'd pay tax. Most of what I get through my aunt's address are tools...sometimes there's tax, sometimes not.
Don't quote me, but both myself and my father have ordered through her, sometimes there's tax, sometimes not. Maybe it's the type of purchases involved.