I say go with the PO Box. Sure they can go to the trouble of getting a warrant and finding out your real name and address from the post office, but it makes them have to got to the effort.
Truthfully, if your seeds are detected, they will just get confiscated by customs and they will send you "the letter". The po po aren't likely to go to the trouble of setting up a whole sting operation for a couple packs of seeds...
Just my .02
Good luck!
~Rock~