Design fundraising experiment and guardrails

You plan to A/B test two online solicitation variants for the nonprofit's email campaign (subject line + suggested amount). Assume baseline conversion is 5.0% and Variant B is expected to improve conversion by 10% relative. Per-person reach cost is $1, and the average donation among converters is $80 (assume unchanged by variant). There is no capacity constraint online. Tasks:

1. Define the primary success metric and at least two guardrail metrics that protect long-term health (e.g., unsubscribe rate, complaint rate). Justify each.
2. Calculate the minimum sample size per arm to detect the expected lift (two-sided test, 95% confidence, 80% power). Show the formula and numeric result; state any approximations.
3. Describe how you would segment results by donor tier (H vs L) without inflating false positives. Include your multiple-testing or hierarchical modeling approach.
4. Outline a stopping rule and the risk of peeking. How would you handle uneven email deliverability across segments?
5. If Variant B raises conversion but lowers average gift by 5%, show how you would recompute the decision using net revenue per reached recipient.