I have an example for France.
La Poste (the national mail service) offer a service called “Lettre recommendée” id est, you send a mail (actual paper mail) to someone or some organization, and “La Poste” do not give the mail without a proof of identity (usually the national identity card or a passport). Wikipedia tell me that this is called a Registered mail in English.
You can imagine letsencrypt sending a token to someone using a “Lettre recommendée” (via “La Poste” API), delegating the proof of identity to “La Poste”. Then letsencryt get the token a couple of days later, it knowns that “La Poste” has check that the identity in the mail is correct. Letsencrypt just has to make people paid the cost of the “Lettre recommendé”, currently 5,64 €.
Of course, that suppose to trust “La Poste” for identity validation, and that may not be possible…
I don’t known, but, how are actual proof of identity made for OV and EV certificates ?
Édith; for the record, you can send “Lettre Recommendée” using this online form.