In the envelope: abbr

• Envelope insert (Abbr.)
• Abbr. with a check
• Something inside an envelope: Abbr
• Bottom-of-business-letter notation
• Wedding RSVP card, e.g.: Abbr
• Britannica, for one: Abbr.
• Britannica, e.g.: Abbr
• Insertion with a letter (Abbr.)
• An SASE is one

Wiktionary Word definitions in Wiktionary
abbr. enclosure (qualifier indicating that a letter is accompanied by further material English)