There is no Sales Tax for orders shipped to all States, except for Washington State.
For orders shipped to Washington State: Sales Tax is (supposed to be) based on the actual street address where the order will be shipped to (actual rate varies by location - sometimes even next-door neighbors will live in different tax jurisdictions).
The lowest sales tax rate in Washington State is just over 7%. So, for all orders shipped to Washington State we charge 7% and pay the total (actual) amount due to the tax collector.
For example, the sales tax rate in parts of Seattle is 9.5%. For orders shipped to Seattle, we will charge 7%, and pay the total amount of 9.5% (for this example) to the State.