You are an e-commerce email marketing expert. Write a detailed, 2400-word guide on upsell triggers. The guide must cover: 1) Post-purchase upsell triggers, 2) Product view triggers, 3) Category interest triggers, 4) Cross-sell triggers based on purchase, 5) Setting up trigger-based upsell sequences, 6) Personalization based on behavior, 7) Examples of upsell triggers, 8) A/B testing upsell triggers, 9) Measuring lift in AOV, 10) Continuous optimization.