.....Vancouver, you raise a most valid point whether or not the post office in Seattle would even rent you a box for a month with your being a Canadian....I don't know....
.....the next time you are there, you could ask an employee of the postal service sorta' tongue-in-cheek, know what I mean?....I am dedicated to the adage, "ask not, get not"....wish I could help more....and probably didn't help a bit....
.....but let me ask you this....can a person/resident in the US send a fairly small box to someone in Canada and the box not have to be opened/checked at customs as long as it's not ticking?....seriously....
.....and did you notice Farscaper's post above?....you might check into that and be done with this issue quickly....
.....I'll never forget the suspension bridge we drove across after entering Canada on the way to Vancouver.....
.....I'll also never forget the young woman's eyes at the border check....she looked out that booth window with the eyes of a Federal Judge, and looked into me all the way down to my feet when she asked me if I had anything to declare....