Hi, is there a post office in Plain WA?
According to wiki it closed in 1936 lol. It's less than 20 miles to Leavenworth though. Which has one, and I'm going to assume you won't be without a car!
I don't remember seeing one when we stayed there, but you might contact Plain Hardware as it is a general store/ski rental place and they are pretty much the heart of the community. That said, I think Marie's suggestion of Leavenworth is probably your best bet.