Mail theft

So, I wonder who I call about postal mail theft. Its fairly obvious that our upstairs neighbors are stealing our mail. My roommate has lost checks and actual money. I am now missing a letter (from an envelope which looks like a check would be in it.)

I've never had worse neighbors. Ii really do want to leave a flaming bag of poo at their door. But we share a stairway...so I'd have to suffer as well. Damn.
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