A postscript is added at the end of an article or letter. It comes from the Latin words "post" meaning after, and "script" meaning writing. A postscript to a letter starts with the letters P.S. which stand for postscript, after writing.
Elizabeth always saved the juiciest gossip for the postscript in her letter, knowing that every reader would read everything else before getting to the best bits, which she saved until last.