Unfortunately, the customer_id isn’t available for $0 orders. If you pay with a card it will be available. Also if you add a fulfillment to the item the collected customer information will be in the order fulfillment object. 
Unfortunately, the customer_id isn’t available for $0 orders. If you pay with a card it will be available. Also if you add a fulfillment to the item the collected customer information will be in the order fulfillment object.