Using an MQ with a payment processor is very common - payment processors do have transient outages and we still want to allow customers to transact. We may also

Using an MQ with a payment processor is very common - payment processors do have transient outages and we still want to allow customers to transact. We may also use multiple payment processors in this case and fallback to a more expensive one if we see our preferred one go offline for a while. MQ's are distinct from APIs. APIs require both sides to be free to talk, while an MQ allows one side to send when it's able and the other to read when it's able which can happen out of sync. MQs do have limits but this is typically several orders of magnitude higher than an API - a few million queued messages is pretty trivial on any decent MQ cluster. An MQ isn't a buffer alone, it's also routing, retries, error handling, deadlettering, etc. This is the difference between your crappy in-memory blind buffer and something like RabbitMQ. Yes MQs still require maintenance/attention/etc, they're not magic.