Related Clues:

• Abbr. on a copier paper tray
• SASE, perhaps
• Wd. component
• Monogram pt.
• U.S.P.S. delivery
• Ltr. extras
• Business letter abbreviation
• Brief correspondence?
• SASEs, e.g.
• Envelope contents: Abbr.
• Word unit: Abbr.
• A, B, or C: abbr.
• USPS unit
• Env. insertion
• Extra things in the env.