~120 mile point should be between the Winlock and Vader area. Looking at Cascade's site, there is the high school which charges $40 but seems worth it because they provide accommodations, shower, baggage service, dinner, etc. To me that sounds totally worth $40!
http://cascade.org/EandR/stp/stp_centralia.cfm
You might want to contact them to see if they'll allow camping outside on if you want to go the free route and skip the other amenities.
midpoint baggage service =>
http://cascade.org/EandR/stp/stp_baggagesvc.cfm