I'm actually kind of surprised this town has a post office since most the population fluctuates so rapidly due to the fruit picking season and agriculture. Otherwise, it is a very nice post office right nest to the Asian/Mexican restaurant-grocery. It doesn't have an automated postal center so you basically have to get everything done while they are open.
If you're in the area, Mattawa is not that far from the beautiful views of the Columbia River and if you're a science geek, or not so much, you can get close to the Hanford Nuclear Site whose B Reactor plutonium was used for the first nuclear test in New Mexico as well as the plutonium in the "Fat Man" bomb that was dropped on Nagasaki. You have to register online if you want to go on a tour though as there are a limited amount of people that can go and specific dates on which tours are given. read more