I would bet a neighborhood kid or someone is taking it out of your mailbox.
I have family and friends that work for the postal offices and can tell you that a "used" DVD is not worth losing their salary, awesome benefits package, conglomerate amount of vacation time, holidays, etc. Not to mention it's very difficult getting a position (route) in the first place.
Postal workers have very strict policies when it comes to theft and reports of such are investigated thoroughly.
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