Mike,
I am an American, living in Scotland. 15 digits sounds like a lot, unless that includes the p.o. box. i.e. sometimes the p.o. box is repeated in the zip code. Andrewm is correct. Usually the zipcode has five numbers, indicating the town and then a four digit number denoting the exact location. Usually (if I remember correctly) if it's a p.o. box that number will be repeated in the four digits. I appreciate your desire for confidentiality, but I could probably give a more informed answer, if I see the full address. I know you don't know me from Adam, but if you PM me with the details, I promise I won't and have no reason to use the information for any other use than to help you. I'm thinking as well, sometimes large organisations in the U.S. have something like a barcode at the end of the address. Does your friend work for a large organisation, and is he using his work address? Another thing to think about, some p.o. boxes will not accept parcels.
Brad