Yes, there are some offices in Seattle and its neighborhoods that are open till late that is, beyond their regular timings.
http://questionland.thestranger.com/questions/910-are-all-post-office-branches-open-till-midnight-for-mailing-taxes
Some of these include:
1. Seattle, Riverton Station: This post office offers retail and pick up until midnight
2. Tacoma P&DC: This post office offers curbside pickup until midnight
3. Bellevue Main Office: The post office offers pickup until midnight
4. Bellevue Midlakes Station: The post office offers pickup until midnight
5. Bellevue Crossroads Station: The post office offers pickup until midnight
6. There are some other post offices as well that offer pick up services until midnight. These include Bothell Mill Creek Station, Edmonds Main Office, Everett Processing & Distribution Facility, Lynnwood Post Office, Olympia Main Office, Olympia Tumwater Station, Olympia Westside Station.