Why am I still losing money due to RDS, even though I stopped and deleted it?

Hello.

Is it possible that you are checking in a different region?
When you check your bill, you can check in which region the bill is being charged in the RDS field.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

Also, a common reason why you are being charged for deleting an RDS instance is that the automatic snapshot created at the time of deletion remains.
So, from the snapshot screen for all regions, check all the tabs in the red frame in the image below to see if any snapshots remain.

You can also check which RDS features are being charged for by sorting from Cost Explorer as shown in the image below.