Obviously then, the paragraph headings are extremely important. The use of blank lines and headings between the paragraphs simply makes it easier to read. You should also be aware that there are many different line lengths used in email and in publications. Many times if your line length is too long, it will not wrap properly and you will have long and short lines, giving a jerky look, instead of ones which are uniform. Your best bet to avoid this is to use a line length of 60 or 65 characters, as even older email readers can read these lengths properly.
Most people read from left to right.
It would therefore seem logical that all the text in your ad is aligned to the left side of the page. However - we are not talking about a lengthy document here. If your lines are short, and consist of only 4 or 5 lines, it might make more sense to center them on the page. This is a good idea if you are placing your ad in an ezine or newsletter. The text in the publication is left justified, and your use of a centered text ad will make it stand out.