Places where SWAK may be written
Else clues• GPO purchases
• Items that get stuffed and stamped: Abbr.
• Letter carriers (abbr.)
• Ltr. carriers
• Ltr. holders
• Mailers: Abbr.
• Pt. of SASEs
• Stationery store stock (Abbr.)
• They may be self-sealing: Abbr.
• They're often sent by the USPS
• Where to find the theme entries of this puzzle, briefly