This is incorrect; you can indeed get Informed Delivery for [at least some] P O Boxes, postal employees don't know how to do it, but if you change the address on your Credit Card to be the P O Box, and then try authenticating using that card, it passes fine. That's what I did for my own P O Box, for example (it failed the first time when my card was using my street address).

I've been able to get it to partially work for my PO BOX.

For some odd reason it seems to only work for packages. Not standard letter mail.

At least now I get an alert that a package went to the Post Office and isn't coming to my house.