Write like you speak. Your sales letter isn't the great American novel, so don't write it like you are Ernest Hemingway. Use short paragraphs that lead into the next paragraph. Don't be afraid to use short sentences. Like this one.
Shorter paragraphs keep the interest of the reader.
Longer paragraphs cause eye strain and often force the reader to get distracted. Stress the benefits, not the features. Spell it out. Don't focus on the features of your product or service , but rather how those features will add value to the life of your reader. I don't care about your selection.