When I bought my HPWC from the Tesla website I got charged sales tax. I'm not sure where to send the form when I fill it out. When I contacted the only electrician recommended by Tesla in the Portland metro area, he sent me a tax incentive form for Oregon and it appears he wasn't aware of the Washington program.
It's small compared to the sales tax paid on the car, but it's still going to amount to about $160 for me.