Your issue will not be the ability to drive in Canada, it will be to find a car rental
company that will allow the rental of a car in Kelowna to drop off in Seattle.
I searched the companies that are available at the Kelowna Airport...They are Avis , Budget and National. (Links open in a new window)
First I checked availability for a car rental to pick up in Kelowna and drop off in Seattle. It was not allowed with any of the rental agencies. In my experience, the only other option to try was a one way rental from Kelowna airport to the Vancouver airport and then a seperate one way rental from the Vancouver Airport to the Seattle Airport (this is more commonly allowed).Again, I checked availability with all for that option; the only company that shows availability to do that would be Avis . You would have to book a car from Kelowna (YLW) to drop off at Vancouver (YVR). Then you would have to book a seperate rental to pick at SEA to drop off in Seattle (SEA).