Possible Clues:

• Letter-ending abbr
• 'Another thing I forgot to mention ...': Abbr
• Certain letter addendum, for short
• Additional afterthoughts: Abbr
• Forgetful writer's letters?
• And another thing, on a letter
• Letter addition letters
• Second afterthought on a ltr
• Second afterthought: Abbr
• Ltr.'s second add-on