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I was at that same post office this morning to drop off some return packages. I asked the clerk about this. She said that they get a lot of IDs(including passports), wallets, and even purses dropped in the mailboxes.
If the ID has an address on it, they throw it in an envelope and mail it to the person, with a postage required note on it, so the mailman can collect the postage. If it's a purse with ID and address inside, same thing... but in a package and the recipient is asked to pay for the standard shipping rate.
With passports, they all get shipped back to the US passport agency, but only once every 2 weeks in a large box. Apparently they get several dozen passports in the mailboxes a month, probably due to the proximity to a large international airport.
