Well, now we are getting into legal hypotheticals. Let me ask you a question: does the process server WANT service to succeed or fail?
After all, Josh could have a real, legit, brick-and-mortar location and a dishonest process server could claim they attempted service there but it failed. So if someone files a service failure report with the Secretary of State, what is the process that agency follows? Here in California, the Secretary of State would send a certified/return receipt mailing to the Agent for Service of Process, which would be received and successfully answered in the USPS scenario.
Yes, you are correct that if you walked into a Post Office, shouted a name, and threw a pile of loose papers on the ground, that would not be handled by postal employees and likely end up in the garbage, not a PO Box.
However, to counter your point, a FedEx envelope is also not mail. But a USPS employee will accept it because it is properly addressed.
So if the process server actually wants service to succeed because that is what they have been paid to do, they will minimally put it in an envelope with your name on it, which I can confirm USPS employees have accepted and have put in my PO Box.
Also in California, when service fails, the Secretary of State accepts service on behalf of the entity. As long as the entity checks in with the Secretary of State once every two years, any errors in the Statement of Information are forgiven. The only time I personally have experienced a corporation being terminated due to service issues was when the certified mailings sent out were returned undeliverable for a period of several years.