RDS keep costing even if I delete all instances

Hello.

If you look at the image of the bill, it looks like the charge is for an RDS snapshot.
However, if you look at the other image of the RDS dashboard, the number of snapshots is 0, so it seems strange that a fee is being charged.
Just to be sure, please check if there are any snapshots left in the tabs circled in red in the image below from the snapshot screen.
If any snapshots remain, delete them.

If you cannot find the snapshot, I recommend that you open a case with AWS Support under "Account and billing".
Inquiries under "Account and billing" can be made free of charge.
https://docs.aws.amazon.com/awssupport/latest/user/case-management.html