what is the reason that some people wouldn't ship to PO Boxes or military APO/AFO addresses? if signature confirmation is available, why does it seem like some sellers don't want to ship to them? are there other risks involved?
You can absolutely have signature required to a PO Box if using UPSP Priority Mail. I have a PO Box and when I get a package, they leave a note in my box that I take to the counter. The counter person gets my package from the back and I sign for it.
Some people may not know this and that's why they're hesitant to ship to a PO Box. (?) I don't know about APO/FPO boxes.
Also if the shipper uses FedEx, UPS or DHL, they do not deliver to PO Boxes - they need a physical address instead.