AWS Bandwidth Cost

Does customer use Load Balancer or NAT Gateway (though... if they have Elastic IP on EC2 instance, I assume there's no NAT Gateway?). If they have Load Balancer or NAT GW in the same AWS Region, but in a different AZ from that EC2 instance, and if either of these two communicate with that instance, then there would be Regional data transfer charges.

Or, are either of those resources in the same AZ, but they communicate with EC2 instance using Elastic IP? In this case they would be also charged for Regional data transfer (inter-AZ traffic).

Another thing could be... the EC2 instance communicates with S3 bucket in the same region, but using Elastic IP. In this case customer would still get these charges. A better option would be to use VPC endpoint and have EC2 instance connect with S3 through the VPC endpoint (not using Elastic IP).

Take a look at this blogpost that talks about data trasnfer charges in common architectures - it may be helpful in identifying better way to implement this for your customer: https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/