For shipping 1-4 guides (depending on thickness) I use a plastic #5 bubble mailer, which keeps guides from getting wet. I pack cardboard top and bottom over the stack, which is what you're doing with your guides, and that helps them from getting bent in shipment. I found the #4's were a little snug for some of the bigger/thicker guides, like the hardback LE's. I also mark the bubble mailer on both sides with DO NOT BEND in large print, as some postal carriers or workers will bend a bubble-mailer package, unless they're told not to. The cardboard helps prevent that as well.
For more guides than would fit in a bubble mailer, I use a box and pack it with some packing material, such as paper, airpack or bubble wrap. Some of the larger sized Amazon.com boxes work well in that respect. If I've received guides from another CAG, I recycle the box and use it again, if it's in good shape.
Shipping is media mail w/DC, unless otherwise requested for priority mail. If priority mail, then the flat-rate mailers are pretty good for large numbers of guides.