Most (all?) P.O. Boxes in the US have unique zip+4 addresses so you should, in theory, be able to send a letter to: 12345-6789 (assuming that actually corresp

Most (all?) P.O. Boxes in the US have unique zip+4 addresses so you should, in theory, be able to send a letter to: 12345-6789 (assuming that actually corresponded to a P.O. Box) and have it delivered. With automation, you might even be able to just put the bar code for a letter on the envelope and have it delivered. I say this based on the fact that I used to stamp mail for previous inhabitants with a not at this address and black out the address (but not the bar code), dump it in the mailbox and then have it show up in the email I get daily from the post office showing the day’s mail (fortunately, the human step of delivery usually pulled those letters out of the process).