According to
dor.wa.gov/docs/Pubs/ExciseTax/RetailSales_UseTax/RetailSales.pdf:
QUOTE
sales tax exemptions
Some of the most common exemptions
from the retail sales tax include:
Sales to Nonresidents:
Sales of products in Washington for
use outside Washington to residents
of states, Canadian Provinces, or
US territories that impose a sales
tax of less than three percent
may be exempt. (See Excise Tax
Advisory 2014.08.193 for additional
information.)
Interstate and Foreign Sales:
All sales of goods delivered to buyers
outside Washington are exempt from
the retail sales tax. Likewise, sales of
items for use in Alaska, Hawaii, and
noncontiguous US territories are
exempt, if delivered for direct
shipment out-of-state.
(WAC 458-20-193)
END QUOTE