Note, Washington State Senate Bill 5061 seeks to block certain "untracable" firearms without a serial number. This relates in part to "3D printable" guns, but the act of milling of a 80% lower (in Washington) could be a violation, if and when it passes. But it appears the bill hasn't been made a law yet... Is Washington specific news this issue what drives the question?
Federally, the ATF web site says:
Receiver blanks that do not meet the definition of a "firearm" are not subject to regulation under the GCA. The ATF has long held that items such as receiver blanks, "castings" or "machined bodies" in which the fire-control cavity area is completely solid and un-machined have not reached the "stage of manufacture" which would result in the classification of a firearm per the GCA.
But this gets a bit technical. For further risk mitigation, the Texan could mitigate any risk related to both Federal and Washington state law by shipping it to a federal firearms licensee (FFL) in Washington as if it were a firearm, per ATM instructions which say (in part):
Generally, for a person to lawfully transfer a firearm to an unlicensed person who resides out of State, the firearm must be shipped to a Federal Firearms Licensee (FFL) within the recipient’s State of residence. He or she may then receive the firearm from the FFL upon completion of an ATF Form 4473 and a NICS background check.
That FFL is then responsible for ensuring the transaction properly conducted in the state of Washington, including federal and state requirements. The FFL I contacted only charged $25.00 (plus collecting the state sales tax). Form 4473 was easy enough; only about one page for me to fill out. Of course, I've got a clean record, so going through channels isn't a problem for me, it took less than an hour.
The FFL confirmed for me that it wasn't necessary for something like an antique musket, which legally isn't a firearm by the federal defeinitions. In my case the sender was a nervous "trust" lawyer who wasn't sure, hired yet another lawyer to advise him. The FFL didn't charge me anything for receiving that musket.