Anything inside an envelope: Abbr

• Ltr. add-in
• Env. insertion
• "Britannica," for one: Abbr
• Accompanier of a letter inside an env
• Wedding RSVP card, e.g.: Abbr
• SASE, e.g
• Letter-ending abbr.
• Collier's work: Abbr.
• Letter-ending abbr

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