Originally Posted by
wandering_fred
Or more correctly neither USA-CANADA-USA nor CANADA-USA-CANADA flight combinations can be ticketed as a single ticket due to the respective cabotage laws.
As suggested take surface transport to SEA and buy your ticket from there.
Happy wandering
Fred
Actually, US-Canada-US can take place provided that both segments are operated by a US airline. Indeed, US Air used to regularly publish and sell BOS-YYZ-CLE schedules on its own equipment.
However, since no Canadian airline is a OW member it is not possible for OP to fly a Canadian carrier via the US and receive OW miles. OP could, however, purchase YVR-SEA-YYZ on AC (not UA operated flights) and receive AC miles; but, AC is not in OW.