As noted above, ZIP+4 uniquely identifies most (all?) US PO Boxes, so theoretically provides everything the post office needs to make the delivery. For what it

As noted above, ZIP+4 uniquely identifies most (all?) US PO Boxes, so theoretically provides everything the post office needs to make the delivery. For what it's worth, I've personally mailed an envelope addressed only to my PO Box ZIP+4 with no return address, and it worked. Note, however, that abbreviated "addresses" of this type violate USPS's published guidelines, so they may be rejected by automated mail sorting systems and therefore delayed, or, especially in cases where discounted bulk rates apply, rejected altogether. In other words, YMMV.