When cancelling a service, you have two options:
End of Billing Period: The server remains active until your paid time runs out. (Recommended).
Immediate: The server and all data are deleted instantly. This cannot be undone.
What happens to billing: once a cancellation request is accepted, no further invoices are generated for that service. You are not charged again while you wait for the cancellation date to arrive. Your service simply stays active until the date shown on it, and then stops.
Any invoice that is already issued and unpaid at that point is no longer required, and the service will not renew.
If you need help with this, please submit a ticket.