Setting an SLO and Alerting on Burn Rate

You own a checkout API doing 50 million requests per 30-day month. The product team wants "99.99% availability" in the SLA. Historically the service runs at about 99.95% success, and your two upstream dependencies each have 99.9% SLAs.

1. Is a 99.99% SLA achievable? What SLO and SLA would you propose, and why should they differ?
2. For your proposed SLO, compute the monthly error budget in requests and in minutes of full outage.
3. Design the alerting: what SLI, what burn-rate thresholds and windows, and what should page versus ticket? Explain why you would not simply page on "error rate > 1%".