Define a return first
Set the qualifying event and attribution window before sending. A reply is different from an attended appointment or a paid bill. Count each customer consistently and avoid double-counting a bill.
Include the costs
Subtract messaging charges and redeemed offer costs from the attributed revenue estimate. If you are evaluating profit, include the cost of delivering the underlying product or service as well.
Ask what would have happened anyway
Some recipients would have returned without a message. Where practical, compare with a randomly selected eligible holdout group. Explain limitations and avoid treating all attributed revenue as additional profit.