For a one-off trip, I would go with a cardboard box. Even if it gets destroyed on the way out (unlikely), it's not like there's a shortage of bike shops in Seattle. If you break it down and fold it in half, it is possible to carry one on a bike for short distances. Also, Alaska Air has one the best policies for bikes. Fee is $50, and they don't double dip for overweight and oversize. UPS will be at least that and probably more depending on the dimensions.

Definitely get the plastic dropout spacers and hub protectors (free at a bike shop). If you're OCD about your paint, wrap the frame in the large bubble wrap or pipe insulation.

edited to add: As long as nothing is moving around in the box, and everything is lightly padded, there's not much chance of damage. Because of the large surface area, the box does a good job of protecting the bike from stuff getting stacked on it.