Whether you or your customer need to cancel a scheduled job, you can easily do so in the web portal.
Canceling a job (from the web portal) will give you an option to notify your customer that the job has been cancelled (if the customer notifications are turned on).
1. Open the job details page
2. Click on the gear & wrench icon
3. Select 'Cancel job' from the drop down
4. Click 'Yes, cancel this job' to confirm
The checkbox next to 'Notify customer' will default to the customer's notification settings