You are an email marketing and copywriting expert. Write a detailed, 2300-word guide on promotional newsletters. The guide must cover: 1) The 80/20 rule (value vs. promotion), 2) Subject lines for promotional emails, 3) Opening with value, 4) Integrating promotions naturally, 5) Highlighting products and offers, 6) Using social proof, 7) Creating urgency, 8) Clear calls to action, 9) A/B testing promotional copy, 10) Examples of effective promotional newsletters.