AWS Snapshot Billing Issue ?

Hello.

EBS snapshots are incremental backups, so they create a backup of the parts that have changed since the last snapshot.
This means that you will be billed for the difference in size.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSSnapshots.html

You can back up the data on your Amazon EBS volumes by making point-in-time copies, known as Amazon EBS snapshots. A snapshot is an incremental backup, which means that we save only the blocks on the device that have changed since your most recent snapshot. This minimizes the time required to create the snapshot and saves on storage costs by not duplicating data.

It looks like a full backup is being taken because it is a specification of EBS snapshots, so in reality it is only a differential backup.
You can check the actual snapshot size with the AWS CLI command described in the documentation below.
https://repost.aws/knowledge-center/ebs-incremental-snapshot-creation-slow

By design, EBS snapshots show the full size of the backup instead of the incremental size. The first snapshot is the full copy of your data. Subsequent snapshots are incremental and contain the changed blocks from the most recent snapshot. The subsequent snapshot shows the full size because a new volume can be restored from any EBS snapshot, even though its incremental by design. Each incremental snapshot references the other snapshots in the backend.

You can check the size by running the command below.

aws ebs list-snapshot-blocks --snapshot-id value