Neal -
I regularly send 8x10 prints around the US for juried shows. The prints are mounted on 2-ply boards - I put four prints in an ordinary 9x12" envlope (face to face), and then stamp "Do Not Bend" on the outside. The cost is less than $3 for first class postage.
I also am involved in some print exhanges. These 8x10 prints are unmounted, but they are accompanied by some information sheets. A typical exchange group involves 5-8 people, each of whom can have up to three prints in the package at a time. We try to use USPS flat rate priority mail envelopes - $4.05 to send to any US address.
Louie