I think your idea of using the mail will help. Perhaps you can work it out to mail some stuff from Portland to a hotel in Washington (or from your home direct to the Washington destination). Then mail the tent away as soon as possible after the trip.

I've done a few day self-supported trips with narrow sewups. Your narrow 25's should be fine for some light touring unless you are already a heavy rider. Will your riding around before and after the trip be mainly in one town, when you can use a hotel as a base, or are you doing other tours... such as heading to Astoria for a day?