Related Clues:

• It's sometimes certified
• Garb for Galahad
• Junk in the box?
• Kind of bag or box
• Snail ___
• It may contain a lot of letters
• Four letters?
• Blue box material
• It can be registered
• Viewer ___
• See 46 Across
• Spam, say
• Use the USPS
• Vacationer's accumulation, perhaps
• Letters in a box