People who claim that almost always forget to include the s/h fee in the total price (and yes, Washington state does require this for taxable items: https://dor.wa.gov/forms-publications/publications-subject/tax-topics/delivery-charges#:~:text=Does%....) or to add in the local tax rate (at the delivery zipcode, use full 9 digit one in the shipping address for accuracy where there is more than one tax district in the 5 digit zipcode).
Problems with exempt items being taxed at the full rate usually are due to miscategorization of the listing by the seller.
