You need to reserve your car online and choose airport as your pick-up point for your rental car. From SeaTac airport there are buses that will take you for free to the car rental areas. Where you can get your car. You need to return your car to the airport, otherwise you will pay at least $100 (maybe more) if you return the car in a different site. In my experience, pick-up and drop-off of renter is only within 5 miles radius maximum. Hope this helps.
Not much to add there. You can take light rail up to Everett if you want you use this location specifically
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