We do not have any dedicated category for AWS services falling AWS PrivateLink charges.
Ideally, almost all the charges should fall under the EC2 transfer costs under statement drilldowns.
Data transferred "in" to and "out" from Amazon EC2, Amazon RDS, Amazon Redshift , Amazon DynamoDB Accelerator (DAX), and Amazon ElastiCache instances or Elastic Network Interfaces across Availability Zones or VPC Peering connections in the same AWS Region is charged at $0.01/GB in each direction.
https://aws.amazon.com/ec2/pricing/on-demand/
Data transferred between Amazon EC2, Amazon RDS, Amazon Redshift, Amazon ElastiCache instances and Elastic Network Interfaces in the same Availability Zone is free. See above when transferring data using VPC peering.
Data transferred between Amazon S3, Amazon Glacier, Amazon DynamoDB, Amazon SES, Amazon SQS, Amazon Kinesis, Amazon ECR, Amazon SNS or Amazon SimpleDB and Amazon EC2 instances in the same AWS Region is free. AWS Services accessed via PrivateLink endpoints will incur standard PrivateLink charges as explained here.