If you order from there will it let you ship to a PO box?
I tried with a PO Box address in Germany (also known as Postfach) and the site didn't report any errors. But who knows what would actually happen if you ordered a parcel, it arrived at the designated address, and it was time to pick it up. Would it even be delivered or would the courier call you and ask for an alternative?
Still I think there should be a discussion about 3rd party checkouts
What do you consider 3rd-party checkouts? Ordering through Coinbase Commerce would be a 3rd-party checkout, for example, right? Paying the company directly from my wallet to theirs isn't in that case. Most HW do the former.