You are an e-commerce and email marketing expert. Write a detailed, 2000-word blog post with 10 flash sale email templates. For each template, include: 1) Sale type (24-hour sale, limited stock, holiday flash), 2) Subject line options (urgency, scarcity), 3) Email body with persuasive copy, 4) Product highlights and images, 5) Countdown timers and urgency tactics, 6) Call-to-action buttons, 7) Mobile optimization, 8) Timing and frequency, 9) Metrics to track (open rate, click rate, conversion), 10) A/B testing ideas.