Yeah, IIRC they don't support this feature for PO boxes, but I managed to accidentally get it for mine when I filed a change of address form.

I regularly see notifications for mail really indented for a few other PO boxes. It's pretty clear that their automated sorting equipment is not perfect, and that they're relying on the postman to do a final quality-control check.

This is actually an improvement from several years ago, when I'd actually sometimes get the mis-sorted mail in my box. Now that hardly ever happens.

I use this with my PO box just fine. The problem is that the pictures are taken at the sorting center not at your local post office. So when things get really busy you'll get the picture a day or two before the letter ends up in your box.

For example, the full ZIP+4 of "PO BOX 391234, CAMBRIDGE MA 02139" is "02139-1234".