I'd be curious to know what this other tax is? Since the Volt isn't allowed the sales tax incentive as it's not an EV only. It states on this PDF that the Volt is exempt, that the tax is for those EV only. "The $100 registration renewal fee does NOT apply to a plug-in hybrid electric Vehicle (PHEV) such as the Chevy Volt"
http://www.westcoastgreenhighway.com/pdfs/WA_EV_registration_renewal_$100_fee.pdf
http://www.westcoastgreenhighway.com/pdfs/WA_EV_registration_renewal_$100_fee.pdf