I never mark anything on a box... Seems to work better for me. Although I must say, I've probably shipped out 70 boxes of plants (mostly non-orchids, bare-root) through USPS in the last month, and all of them have made it safely as far as I know (no complaints). I've taken to using the 'click-n-ship' software, which gives you a nice bar-coded label, which I'm sure helps with sorting.
A few boxes to the left coast have taken 4 days instead of three... But that is about as bad as it gets. I pack assuming a week anyway. Always prepare for the worst and hope for the best!