For future Seattle, WA Volt owners' information: The GoodToGo electronic tolling RFID stickers work fine through windshield of the Volt.
Many of today's cars have special coatings or materials embedded in the windshield to reduce IR (infrared) transmission, this helps reduce the load on air conditioning on hot days. The Volt is one of these vehicles, and since some of these materials also block RF (radio frequency) emissions, I was worried we'd have problems.
I tried researching this beforehand but had no luck. (For those not in Seattle: They just turned on the system two weeks ago.) Earlier this week I decided to just go for it and see what happens. Buy sticker + apply sticker + drive across 520 bridge... and wait.... and wait...
The toll deduction was posted on the GoodToGo web site sometime last night.
So it's all good!
Now we know.