IIRC you don't have to pay that in Washington, so you should still be paying the listed price :). Bit of an edge case though
However, this was intended to benefit border cities--Vancouver, WA in particular really liked it--but Washington State has no income tax and so is heavily reliant on sales tax. So, as of July 1 2019, the exemption (on state sales tax; local sales tax isn't covered here) may only be claimed post-transaction by the buyer: https://dor.wa.gov/taxes-rates/retail-sales-tax/sales-tax-ex...