Does your notification have to be near real-time?

If not, I recommend Amazon SQS: - It'll help you to scale - Probably easier to deliver your solution, since AWS solutions integrates really well with each other and is kind of "managed" for you

Keep in mind however that Kafka may be a more robust solution, but you'll have more effort maintaining it.

Also, there's AWS Kinesis, which is a "Managed Kafka".