Related Clues:

• Post office delivery
• It's sometimes certified
• It's inside: Abbr.
• Garb for Galahad
• Junk in the box?
• Letter add-on (Abbr.)
• Kind of bag or box
• Hard-to-break plates
• Snail ___
• Like an SASE, often
• It may contain a lot of letters
• Four letters?
• Blue box material
• It can be registered
• Viewer ___