You may abbreviate enclosure, or you may not. Copies create a different situation in their notation. That means someone enclosed a carbon copy or copy of something or the letter itself. Carbon paper disappeared many years ago. I doubt if you could find carbon paper, except inside some forms around the company.
Postscripts mean exactly what they refer.
You have forgotten to say something in the body of the business letter. You add a postscript or an afterthought. P.S. Don't forget we have a tennis date next week at the Westside Courts. You have to remember that a postscript should be used sparingly. If the sentence belongs in the body of the business letter, then, by all means, place that sentence back in the body.