Fulfillment of products with Shipping not required

We have a store that sells a mix of both warehouse items and digital assets. All the digital assets have a Delivery Method of “Shipping not required”. We are trying to mark the line items as fulfilled on the order via a webhook when the order is placed. When we query the order for fulfillmentOrders via the following graphQL query:

query getFulfillmentOrders($orderId: ID!) {
order(id: $orderId) {
fulfillmentOrders(first: 10) …

we are getting an empty array, and therefore are not sure how to actually fulfill the line item. If somebody could share the proper steps to fulfill “Shipping not required” items via the admin graphQL api that would be much appreciated.

Thanks!