The Evil Mad Scientist folks have compiled an annotated bestiary of junk-mail envelopes that are camouflaged to look like important correspondence. It's a fascinating study in meatspace spam.
Often times, these envelopes are quite well done. Above is an example that might cause a genuine double take— with its “FINAL NOTICE ENCLOSED“ — and bank-PIN style tear tabs on the sides...
And then, there’s the fine print, so that you really take it seriously. A $2,000 fine or 5 years imprisonment(!) are threatened under §1702 should you fail to deliver this fine specimen of
junk mailletter to its intended victim. (This penalty is true but somewhat misleading; the law refers to obstruction of mail in general, not this “final notice” in particular.)