If you're going to drive up from the SEA area to park at YVR, fly from YVR to LAX, so you can drive home to the SEA area on arrival by ship, I'd try doing the O/W car rental per above. In the distant past I've been able to rent from D/T Vancouver locations (Hertz, maybe Avis) for return at SEA which will save you the YVR airport rental surtax.