Interesting idea about drone-driven mailbox-loader! However, it still has the weakness that the mail will go through the USPS, and they have the opportunity to photograph (and weigh?? and X-ray???) it.
I've been following dropgangs as an intellectual exercise for a few years. Even if it doesn't involve advanced concepts like drones, it is an extraordinary advance, mostly made possible by the ubiquitous availability of GPS and other GNSS signals. I would guesstimate that at least 1% of a city or town's surface area would be suitable for placing a drop.