I have both Alliance and Horde characters. One house for each - but only one mail box.
Is there a way to buy another one or get one in a housing endeavor?
Make or buy another one. There are multiple mailbox decor.
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None in endeavors IIRC, but there are a few that can be bought at a vendor (often after a quest) or crafted with inscription/enchanting/leatherwoking.
Search with both MAIL and POST keywords. You’ll get a lot of fences with the last one, but also half a dozen mailbox options.
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If memory serves, doing the campaign questlines should give a couple.