You must have written a letter in your life sometime. But do you remember using a P.S. in the end of the letter ? I remember when I was a kid ,I use to write letters to my cousin in England. Always by the end of letter, I use to remember something that I had forgotten to add in the letter and i would put a P.S. and add it there. But have you ever wondered why a P.S. is needed in Sales Letter?
No doubt in the earlier times when there was no computer, P.S. (postscript) was used to add something if you had forgotten to add it in the sales letter. But why now, when you can easily edit anything on your computer? The answer is to bring the reader's attention to an important point. It is not an afterthought, it is a purposeful part of the sales letter. P.S. is always put at the end of the sales letter reminding the reader of the deal or special offer or urgency with at time limit or reminder about limited supplies.Here's an example from a letter for a health insurance company in California:
P.S. Supplies are limited. Please respond by December 31 to request your FREE copy of The Complete Guide to “No Surprises” Medicare Health Plans with no obligation. 
Do you think P.S. in a sales letter makes any difference ? Let me know if you have a different view.