As a former resident of WA let me share with you what happens there: there is a state sales tax plus each county has its own sales tax. Odds are there are parts of the country cities and transit districts also have sales tax. Oh, and shall I add that local rates in particular change all the time?
Then there are a few outliers, like Oregon, that have no sales tax.
The background noise on all this is Federal legislation that would not exempt retailers from collecting (and remitting) sales tax as in the past. It would make life easier if states with sales taxes would be consistent throughout the state but I don't think the Feds can mandate that.
This isn't just an Amazon issue. Unless the legislation exempts retailers below a set annual dollar volume of sales this could be an administrative nightmare for all internet retailers.
Avalara: Sales Tax Rates, Exemption Certificates and Filing Software