I just found (here and here) it very surprising that, apparently, in the US you cannot just write a letter and drop it in someone's mailbox: even if you deliver it yourself, you still have to pay postage:
18 U.S. Code § 1725 - Postage unpaid on deposited mail matter
Whoever knowingly and willfully deposits any mailable matter such as statements of accounts, circulars, sale bills, or other like matter, on which no postage has been paid, in any letter box established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any mail route with intent to avoid payment of lawful postage thereon, shall for each such offense be fined under this title.
So, say I want to write a physical letter to my neighbor next door. I don't want to hand it in person or put it under their door. I just want to deliver it to their mailbox.
Does the above law require me to pay postage on it? Or, can it be argued that I have no "intent to avoid payment of lawful postage thereon" as there is no reasonable necessity to use postal service in the first place?
Then, as a marketing agency who finds it more economical to hire delivery persons rather than use postal service, would there be any "intent to avoid payment of lawful postage"? As with the neighbors example, it is simply neither reasonable nor necessary for the agency to use postal service in the first place, so what "intent to avoid" can even be talked about? Even if it was more economical to use postal service, is the agency not within its rights to not give a damn about even the existence of postal service and hire its own delivery personnel just because it fancies so?
How do private courier services operate in the US? Do they obtain permission from the Postal Service to use people's mailboxes? (assuming the parcels are small enough to fit in them)
Finally, as someone who wishes to receive letters like that, what can I do to allow the senders do it without breaking the law? Install a separate mailbox which is not "approved, or accepted by the Postal Service" and clearly marked like "non-postal delivery only"?