Answer: PPS
PPS is a crossword puzzle answer that we have spotted 63 times.
    - Second addendum to a letter: Abbr.
 
    - Epistle appendage: Abbr.
 
    - Ltr. addendum
 
    - Letter abbr.
 
    - Second letter addendum
 
    - Second ltr. addition
 
    - Second letter addendum: Abbr.
 
    - Letter ender?
 
    - Later afterthought: Abbr.
 
    - Second ltr. addendum
 
    - Second afterthought abbr.
 
    - End-of-letter letters
 
    - Second notation at the bottom of a letter
 
    - Afterthought afterthought (abbr.)
 
    - Afterthoughts on a ltr.
 
    - Short after-after-thought
 
    - Letters ending some letters
 
    - Letter-extending abbr.
 
    - Second afterthought in a letter: Abbr.
 
    - Letters at the end of some letters
 
    - Additional epistolary afterthought
 
    - "Oh, and another thing," on a ltr.
 
    - Second ltr. addenda
 
    - Second addition to a ltr.
 
    - Second afterthought
 
    - Letters at the end of a letter, perhaps
 
    - Ltr. afterthought
 
    - "Also, I almost forgot ...": Abbr.
 
    - Second afterthought (abbr.)
 
    - Second afterthought, in a ltr.
 
    - Second afterthought: Abbr.
 
    - Second second thought, for short
 
    - Forgetful writer's letters?
 
    - Letter addendum, for short
 
    - Letter extender
 
    - Third thought? (Abbr.)
 
    - Ltr. extender
 
    - 2nd letter addition
 
    - Correspondent's "Oh, and another thing ..."
 
    - 2nd letter add-on
 
    - After-afterthought on a letter: Abbr.
 
    - Afterthought No. 2
 
    - Second afterthought and this puzzle's theme
 
    - Ltr.'s second add-on
 
    - Second afterthought on a ltr.
 
    - Letter addition letters
 
    - And another thing, on a letter
 
    - Additional afterthoughts: Abbr.
 
    - Additional afterthought, for short
 
    - Second afterthought in a letter, for short
 
    - Certain letter addendum, for short
 
    - "Another thing I forgot to mention ...": Abbr.
 
    - Letter-ending abbr.
 
    - After-afterthought, for short
 
    - Second addendum to a letter, for short
 
    - Bottom-of-letter letters
 
    - Ltr.'s second afterthought
 
    - Letters that further extend letters
 
    - Second addendum to a letter ... and a hint to 18-, 34-, 44- and 62-across
 
    - Here's even more: Abbr.
 
    - Letter writer's addendum
 
    - "Also also ...," for short
 
    - Afterthought's afterthought: Abbr.