You are a subscription commerce and email marketing expert. Write a detailed, 2500-word guide on email for subscription boxes. The guide must cover: 1) Welcome sequences for new subscribers, 2) Pre-shipment and tracking emails, 3) Unboxing and engagement emails, 4) Replenishment and renewal reminders, 5) Upsell and add-on promotions, 6) Referral and friend campaigns, 7) Churn prevention and win-back, 8) Survey and feedback emails, 9) Seasonal and themed boxes, 10) Metrics for subscription boxes (churn, LTV).