I created instructions years back for creating custom priority boxes for Lego sets. I found it on my photobucket account today while trying to share a photo. Thought it would be a mitzvah to share again. Maybe an admin could add this to a sticky here. This should be helpful to those sending sets to other FBTB members or shipping items sold on eBay.
There are three of the larger rectangular size priority boxes this method works with. Skinnier sets obviously work better in the smaller boxes and the thicker sets for the larger. These boxes can be ordered for free from http://www.USPS.com
Since I'm a green kind of guy, I figured out you can create three longer boxes from one standard box that's been cut up into three pieces. You get the typical two end "cap" pieces as appears in these instructions and then you can custom trim the leftover middle piece to size and fit it in between another another standard box that is cut in half.
I tried linking the images but they get cut off for some reason so I've included the link below to the album. Click "more" underneath the image for written instructions.