Related Clues:

• U.S.P.S. deliveries
• Contents of many files: Abbr.
• USPS cargo (abbr.)
• Letter carriers: Abbr.
• Office corr., sometimes
• Mailers: Abbr.
• They may be self-sealing: Abbr
• 34-Across concerns
• P.O. deliveries
• USPS pieces
• P.O. box inserts
• P.O. stack
• Monogram units: Abbr
• Brief missives?
• A and P, e.g.: Abbr