That's strange, especially since every PO box* is in fact a unique ZIP+4, so more than meets the "unique delivery point" requirement. For example, the full ZIP

That's strange, especially since every PO box* is in fact a unique ZIP+4, so more than meets the "unique delivery point" requirement. For example, the full ZIP+4 of "PO BOX 391234, CAMBRIDGE MA 02139" is "02139-1234".