It's an US address that's just located right across the boarder. You just pay a small fee when you pick up your package.
I don't use it though coz I already have a permanent mail box in Blaine. But i know some LYK crafters do. Maybe they can explain better.
I just sign up to here: http://www.thelettercarrier.com/
And when you make online order, use that address as the shipping address. When they got the package, will email you. And I go there (point roberts) to pick up and pay a small fee. Pretty convinenent!