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You can put just the zip and the P.O. box number and it'll get there.

The zip+4 uniquely identifies every address in the US. We're probably nearing a billion addresses, though, I wonder what happens after that?

According to wikipedia, zip+4 does not uniquely identify single individuals unless they are high volume recipients.

sometimes they do, depends on population density.

Well, this was before the +4, and the official address was Rural Route #.