Which AWS service meets this requirement?

Explanations:

AWS Auto Scaling is used to automatically adjust the number of EC2 instances in response to demand, not for sending messages between applications.

Elastic Load Balancing distributes incoming traffic across multiple targets (like EC2 instances) to ensure application availability, but it does not provide a messaging capability.

Amazon Simple Queue Service (Amazon SQS) is a fully managed message queuing service that enables decoupling and scaling of microservices, distributed systems, and serverless applications, making it suitable for messaging between ecommerce applications.

Amazon Kinesis Data Streams is used for real-time processing of streaming data, which is more suited for scenarios involving data ingestion and analytics rather than message queuing between applications.