Q. Once and for all: To abbreviate “postscript” at the end of correspondence, is it best to write PS or P.S.?
A. Chicago style for the abbreviation of “postscript” can be deduced from our recommendations for using periods with abbreviations (CMOS 10.4), which can be summarized as follows: (a) use periods for an abbreviation that consists of lowercase letters or that ends in a lowercase letter, such as p. or pp., a.m. or p.m., and Dr.; (b) use a period for an initial standing in for a name, as in E. B. White; but (c) omit periods from abbreviations that include two or more capital letters, such as US, PhD, and CEO.
So we would advise writing “PS,” no periods—followed by a colon. (If you prefer “P.S.” with periods, you can leave the colon out.)