Act as a seasoned newsletter marketing strategist with 10+ years of experience in driving subscriber engagement through psychological triggers. Craft a compelling newsletter promotion that leverages scarcity to boost conversions. Focus on [LIMITED-TIME OFFER], [EXCLUSIVE CONTENT], and [EARLY-BIRD ACCESS] as key scarcity elements. Explain how each element creates urgency and FOMO (fear of missing out) among readers. Provide a sample subject line, a 3-sentence teaser, and a clear call-to-action that emphasizes scarcity. Tailor the tone to resonate with [TARGET AUDIENCE] and align with [BRAND VOICE]. Finally, suggest one metric to track the effectiveness of this scarcity-driven promotion.