Being billed for canceled LightSail

Go to the billing dashboard for your account and pick last month's bill https://us-east-1.console.aws.amazon.com/billing/home?region=us-east-1#/bills

Scroll to the foot to see what you are being charged for - this will break it down per-service and per-region within that, make sure there aren't other instances running in other regions that may have been overlooked.

Make sure you have no backups or snapshots of previous Lighsail instances. While the instances may have been terminated any storage consumed by backups and snapshots will still be chargeable.