PO Boxes are addresses, too
Today I encountered an institution that refused to accept a PO Box as a legit address. "Send me a check," I said, and they replied, "we need your physical address to do so."
The context: a rebate offers where you buy a thing and you receive a $100 rebate gift-card in the mail. To receive the gift card, you have to fill out some paperwork and send it registered mail with a money order for the cost of shipping the rebate. The gift card comes by mail 30 days later.
The gift card's paperwork says it can be used as a credit card; the gift card itself says it is a debit card. The paperwork says no PIN is required; attempting to use it requires a PIN. Subsequent emails with support result in the suggestion to try the card but press "enter" when prompted for a PIN.
To be used online, the card requires you to associate your name, phone number, and address with the card. The field does not accept PO Boxes.
If you would like to cash out this debit card, the card's terms of service prevent it from being cashed out in any transaction, including ATMs.
If you would like to receive a check for the card's value, you have to fill out the form that doesn't accept PO Boxes.
If your Post Office doesn't offer Post Office Box Street Addresses, a scheme where PO Boxes can be obscured by addressing mail to the post office's street address with the box number as an apartment unit number, you're out of luck.
Better go spend the card on exactly $100 of goods.
I took mine down to the drugstore on the corner and bought a $100 Amazon gift card, since that's more useful to me.