correction above, the 123 is the PO box, the -0 is required to make it work. if your po box is only 2 digits, then you would use -00 first

I have the same issue as the origanal poster, and what works for me is writting my street address, and on the the zipcode portion add a - and the 0, then your PO number..

example
..
123 random street
Cobb, Ca.
95426-0123

the -0123 being the PO box #, where as the 95426 is the actual Zip code.