Here is the throughput requirement for a payment backend serving an Amazon-sized store:

1,000,000 transactions/day ÷ 100,000 seconds = 10 TPS

Ten transactions per second. Less than the hotel reservation system, which was already the smallest number in this series.

And payments are harder than either. Because the failure mode is not a slow page or a stale metric — it’s charging someone twice, or taking their money and not paying the seller. There is a real person, a real bank statement, and in many jurisdictions a regulator.