Related Clues:

• End-of-letter abbr
• Business letter abbr
• Insert of a sort: Abbr
• Abbr. on the bottom of a business letter
• Something in an env
• Britannica, for one: Abbr
• Check within, say?: Abbr
• Inside, for short
• It needs to be addressed: Abbr
• Postal abbr
• Env. stuffing
• S.A.S.E., e.g.: Abbr
• Ltr. encloser
• Env. within an env., perhaps
• P.O. purchase