If you are talking about PO Boxes then this is a common flaw. The inside of the post office has boxes and that is a mail route with a unique zip code. When that batch of mail gets delivered to the boxes they are supposed to indicate in the system its delivered but many times the delivery person forgets to put a notification card in the box. If you have a tracking number take that to the front desk (yes stand in line) and ask where the mail item is since it says delivered. They should scan each item that goes to each box but the dont they just scan the whole batch that will be delivered for the day and then forget everything else.
Give them a call. Or if you know when your mail is delivered, check with mail person
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