Global-DataTransfer-Regional-Bytes data transfer charges when using rds and ecs

Hello.

1. What is cost for Data Transfer -> Global-DataTransfer-Regional-Bytes. As I can see it's within Free Tier but only for 1.0 GB. What is the price for 1GB after we exceed free tier limit.

"Global-DataTransfer-Regional-Bytes" probably includes communication across availability zones within the same region.
https://aws.amazon.com/jp/blogs/networking-and-content-delivery/understand-aws-data-transfer-details-in-depth-from-cost-and-usage-report-using-athena-query-and-quicksight/

Communication between Availability Zones is priced at $0.01 per GB as described in the document below.
https://aws.amazon.com/ec2/pricing/on-demand/?nc1=h_ls#Data_Transfer_within_the_same_AWS_Region

Is it possible that this extra Global-DataTransfer-Regional-Bytes is being charged because mine ECS service instance call an RDS (Postgres)? Both ECS and RDS are in the same AZ and same region but on different subnets. Would it help to use Vpc Endpoint?

I don't think there will be any charges if ECS and RDS use the same availability zone.
For example, if you enable a multi-AZ configuration with RDS, there is a possibility that communication with another availability zone may occur, which may incur charges.
You don't need to use VPC endpoints to communicate from ECS to RDS instances.