A generation ago, no one even considered adding a postscript, or P.S., to a business letter. "Postscript" means "after the writing" and was used only in casual correspondence between family and good friends, such as when the writer has finished the letter and then thinks to add, "The kids say hello to your kids!"
In business correspondence, if the writer thought to add something else, he would go back and add it into the body of the letter, rewriting as necessary. Today, a P.S. is often purposely added to business letters, and the use of a P.S. in formal emails and direct mail adds a personal touch or calls attention to an idea and prompts the reader to take action.
A P.S. always goes at the end of the letter, after the closing, signature and title of the letter writer and the name of the company (unless the letter will be printed on letterhead).
Example of a postscript in a business letter:
Sam Spade, Detective
SS Detective Agency
P.S. For my best customers like you, my services are discounted an additional 20 percent today only!
Of course, Spade could easily have included the information that's in the P.S. in the body of the letter instead, but purposely putting it in a P.S. will attract attention. He added "for my best customers" to make it seem as if it's only for a select few, and others won't get that P.S., whether or not that's accurate. Adding "today only" makes the offer urgent. Spade will remove the P.S. from future letters when the offer is no longer valid.
The term "formal email" is a bit of a misnomer since email is a much more casual method of communication than sending a letter. Therefore, it's almost always appropriate to add a P.S. to an email. The P.S. should be in the same position as the example for adding a P.S. to a business letter that you print out and mail.
It might not be appropriate to add a P.S. to an email about a serious subject, such as a legal matter or writing/responding to a complaint letter. You'll need to use your judgment as to whether it's a good fit or seems out of place. It's possible to write a serious P.S., though, such as:
P.S. I truly hope we can solve this issue because I enjoy working with you and would like to continue to do so.
Since the purpose of direct mail is to sell products, services or ideas, most direct mail pieces include postscripts. A P.S. gives the writer one more chance to make her pitch. Direct mail is a busy format that usually includes underlining and highlighting key phrases and important points, even when the subject is serious, so a P.S. is expected and necessary because it stands out from the rest of the mailer.
Example of a direct mail P.S.:
P.S. Don't delay! We have less than 200 remaining of the most popular styles and colors! Call today!
It's common for direct mail to include lots of highlighting, italics and underlining as well as a P.S., and people expect to see one. So, take advantage of that and be sure to add a P.S. for your most compelling add-on thought. In general, always make sure to match the tone of any P.S. you use to the tone of the rest of your letter, email or direct mail. It should stand out by its position at the end and not because it seems awkward.