I have a monthly charge for $9.99 and I'm not sure what it is for
I am not sure which device this is about.
Question marked as
See: How to cancel a subscription from Apple - Apple Support
Another possibility is that it is for an upgraded iCloud storage plan. See: Downgrade or cancel your iCloud+ plan - Apple Support
If the above doesn't help, be aware that this is primarily a user-to-user support forum; other Apple users like you. We don't have access to information about charges to your account. See each of the steps in the following instructions. They include information on contacting Apple Support if the other steps don't help: If you see 'apple.com/bill' on your billing statement - Apple Support or go to Subscriptions and Billing - Official Apple Support