Washington is requiring FFL's to collect the USE tax for out of State Purchases. This is the Sales Tax that you would have paid had you purchased the gun at a local business and which you were supposed to remit to the state voluntarily (but no one does).
In an interstate transfer of firearms the Washington gun dealer is required to collect retail sales tax from the Washington resident on the purchase price of the firearm. The retail sales tax is collected from the Washington customer at the time the customer takes possession of the firearm. Sales tax is collected on the total selling price, including freight and/or delivery charges and other amounts added, such a an amount for insurance coverage.
If the selling price is not evident, it is up to the gun dealer to obtain this price by either requiring the purchaser to show the purchase price or to obtain that information from the out-of-state dealer. If, for whatever reason, the dealer is still unable to obtain the original purchase price, RCW 82.08.010 provides that the fair market value shall be used. It is the responsibility of the Washington gun dealer to determine the price in order to report and pay the sales tax due.
If the firearm is obtained by gift, the instate dealer is obligated to collect use tax on the fair market value of the firearm, unless the receiving person can document that the prior owner had paid retail sales or use tax on the firearm. RCW 82.12.020(3).
However, sales tax does not apply on the service charge for processing the required Federal and State forms and contacting the National Instant Criminal Background Check System (NICS) when the fee is stated separately from the selling price of the firearm and freight and/or delivery charges, insurance, etc.