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You are charged for the EBS volume you provision, not storage used.
When you provision an EBS volume all of that space is reserved for you. You create a volume, your operating system formats it and has exclusive use of it, and probably zeros out most of the space. It's solely allocated to you, so you pay for it.
This is different from S3 and EFS, where you pay for only what you use. The AWS free tier gives you 30GB free, so you shoudln't be charged. You may be charged for snapshots of the volume though.