envs Where to find the theme entries of this puzzle, briefly USPS stack They're often sent by the USPS They may be self-sealing: Abbr Stationer's mdse Mailers: Abbr Letter reqmts Letter carriers (abbr.) Holders of ltrs. or bills Stationery store stock: Abbr. Ltr. holders Ltr. carriers Letter reqmts. Mailers: Abbr. Stationer's mdse. Stationery store stock: Abbr P.O. boxful Stationery store stock (Abbr.)