Where to find and delete backup storage?

RDS database backups are "snapshots", visible in the RDS console. If you can't see any then maybe they are in a different region you've used - Cost Explorer will be able to give you a breakdown by region.

Thank you for your answer. I have no any snapshots visible in any region in my Cost Explorer. I have raised an issue to AWS Support, they have confirmed that there are some errors in the billing. It is being investigated for the moment.