I did SEA-VAC earlier this year. You don't need to show a passport when buying tickets. I bought mine online without a problem. I showed up at the station about 15 minutes or so before departure and had enough time to pick up my ticket and seat assignment without rushing. I wouldn't show up much later though. I did have to show my passport briefly at the station, just so the staff knew I could cross the border.

I would check for discount codes for this route- I able to find one for 25% when I traveled back in March.