The first invoice will be prorated; this means, you will be charged for the period 1 October to your preferred billing date. The second invoice you will receive thereafter, will be a full invoice for the selected cycle.
For instance, if you select the 10th as your preferred billing date you will receive an invoice for the period :1/10/19 – 9/10/19 and a full invoice for the preferred billing cycle 10/10/19 – 9/11/19.