Shannon, Buck and I have had two sons in the military, stationed all over the world. We've shipped all sorts of goodies and have always had good luck. Our son who was in the Navy always enjoyed the chocolate chip cookies we sent. Well, he tried to. His shipmates got the lion's share.
What I usually did - in the case of brownies - was to cut the brownies and wrap each one in plastic wrap and then put into a zipper-lock sandwich bag. Stack them as if in the pan and wrap the the whole shebang in bubble wrap.
Put this "mass" into a box and place that box into a little larger box with styrofoam peanuts separating the boxes. Tape closed and mail. Happy brownies. Happy servicemen.
I did the same thing with chocolate chip cookies, except I packed the cookies in 3-pound metal coffee cans before putting into the final box.
Have fun and "thanks" for lightening the load of our servicemen. It means a lot from a mother who has sent two sons to do their duty.