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Oh! Maybe that's a good startup idea - some kind of 'mail is here to be picked up' notification for the USPS, which could then optimize it's route and only hit places it had mail to deliver/pick up from. I go days with no in or outgoing snail mail, but the poor postman has to walk up to my mailbox and check (no flag, it's inside the building).

In a way, Fedex/UPS already do this...