Possible Clues:

• Second afterthought and this puzzle's theme
• Second addendum to a letter (Abbr.)
• After-afterthought on a letter: Abbr
• Addl. afterthought
• Second letter addendum: Abbr
• Correspondent's "Oh, and another thing ..."
• Letter extender
• Letter addendum, for short
• Second addendum to a letter: Abbr.