You have provisioned the free Heroku Scheduler but receive charges for it on your invoice.
These charges stem from scheduler Dyno usage. The Heroku Scheduler Add-on is $0/month, however, it's an add-on that is used to spin up a paid dyno. In the add-on page, you can find this information under "Dyno-hour costs" in the Documentation.
To stop further charges, you can use Heroku Dashboard to either switch your app to free dynos or delete the Scheduler Add-on altogether.