Jack,
For shipment of a single pen where I'm not using 3 day priority mail I use a section of tube from rolls of fabric. They are similar to the shipping tubes, but much stronger and thicker walls. I wrap the pen in bubble wrap, stuff it inside the tube with whatever invoice, recipt, cards, etc and tape a circle of cardboard to each end. I print out a half sheet of paper mailing label and tape it to the tube, (half sheet just about wraps around the tube, and that's one more layer of tape to protect everything.
If i'm shipping without insurance (pen swap w/list members, etc) it costs between a buck and a buck and a half for first class, (depending on pen size/weight, a slim might be just under a buck whereas a Gman might be as high as 1.75), regular mail shipping. Insurance can be purchased for this level of shipping and is pretty reasonable for peace of mind when you're shipping to a client.
Some of those who participated in the Kaweko ink-ball group buy a while back had theirs shipped by this method. Maybe they could comment.