In a letter, 'Postscript' is added-

Correct answer: at the end of a letter.

Postscript (n) (সংক্ষেপে PS):
- বাংলা অর্থ: পুনশ্চ।
- A short remark or message added to the bottom of a letter after you have signed your name, usually introduced by the abbreviation PS:
- An additional remark at the end of a letter, after the signature and introduced by 'PS'.
- an extra piece of information about an event that is added after it has happened.
- It comes from the Latin - Postscriptum, which literally means "written after."
- An extra message is added at the end of a letter after it is signed.
- In other words, "A note or series of notes appended to a completed letter, article, or book."
- চিঠিতে স্বাক্ষরের পরে যুক্ত বাক্যাবলী; পুনশ্চ; অতিরিক্ত বা সর্বশেষ তথ্য।

Source: A Passage to the English Language by S.M. Zakir Hussain.