noun
A note added after the main body of a letter or document. Often used in formal or personal correspondence.
手紙の最後に追って書きを加えた。
I added a postscript at the end of the letter.
追って書きには、次回の会合の日時が記されていた
The postscript noted the date and time of the next meeting.
追伸 is the more common and concise term for 'postscript' in letters, while 追って書き is slightly more descriptive and less frequent.
Compound of 追って (adverbial form of 追う, 'to follow') + 書き (continuative form of 書く, 'to write'), literally 'writing that follows'. The kanji 而 is an ateji for the reading って.