If you're scrambling to get your taxes postmarked by midnight, there are two post offices in the Puget Sound area that will offer options.
*#1: Riverton Station on 32nd Avenue South has a self-service Kiosk where you can drop of your tax forms by midnight.
15250 32nd Ave S SeaTac, WA 98188
*#2: The Tacoma Mail Processing Center on South Pine has a collection box that will accept your taxes until midnight. It also has a self-service kiosk as another option.
4001 S Pine St. Tacoma, WA 98413