There is one truck stop in Kelso at the Hall of Justice, bags should arrive around 3:00 p.m. Saturday. You'll have to pick them up there and then bring them back the next morning for transport into Portland. Cascade has a bunch of info on their website--here's a link to their STP page with lots of info. You may find the ride details link helpful for specifics re: baggage service.
http://www.cascade.org/EandR/stp/index.cfm
Good luck and have fun!
Mary