Related Clues:

• Check, perhaps: Abbr.
• It's added to an env.
• Unsolicited MS., perhaps
• Env. add-in
• "Look inside" abbr.
• Stamped return env., for example
• Placed inside: abbr.
• Something inside an env.
• Bus. letter abbr.
• Letter's attachment (Abbr.)
• SASE, for one
• 125 Across, e.g.
• End-of-letter letters
• Check sent with a ltr., e.g.
• Env. thickener