On March 22, Washington Governor Jay Inslee signed SB 6058, the write-in filing bill. He did delete the last section of the bill, but that part of the bill seems unimportant. Here is his message vetoing part of the bill, although it doesn’t really explain that part of the bill.
Here is the bill as passed by the legislature. It says write-in candidates who file a declaration of write-in candidacy need not pay a filing fee.