This may not work, but this is the best suggestion that I have if you want to contact Sue Bird.  During the WNBA season send your letter to the following:

Sue Bird
c/o Seattle & Storm
351 Elliott Ave. W. - Suite 500
Seattle, WA 98119

Like I said before, there is no guarantee that Sue Bird will even receive your letter, but this method has worked for me in the past with other WNBA players.