Letter notation: abbr

• Abbr. on an envelope
• Env. contents
• Doc. with a ltr
• Stamped return env., e.g.
• Abbr. on a cover letter
• Bus. ltr. insert
• SASE, for one
• Anything inside an envelope: Abbr
• Stamped return env., for example

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