The data in the OP is based on what info I got from the customer support of all those companies. Does their online shop allow you to complete the purchase after selecting a PO box or is there an error or other type of notification that hinders it? Do you know what carrier they work with in the US? The information I got off the Ledger support team was that their packages in the USA are shipped via DHL, and they don't deliver to PO boxes. So it's a restriction set by the carrier company and not the device manufacturer. 

I got redirected to Coinbase Commerce when I imitated a fake purchase just to see where it would take me. It's the first time I hear payments are also processed by opennode.com. Are those Lightning payments maybe or is it a geographical thing that determines which payment processor a customer is redirected to?