What happens when an Air Waves OnDemand product and a product that I’m fulfilling are in the same order?

This is called split fulfillment or split shipment. Shopify is already set up to handle these situations and the Air Waves OnDemand App takes advantage of Shopify’s API to fulfill only the items that we’re printing. In fact, we do not see the other line items on the order at all. When we send the shipping status updates and tracking details back to Shopify, we will only update the line items we’re responsible for. If there are items you are responsible for fulfilling, the order will go into “Partially Fulfilled” status after some of the line items are fulfilled. Once all line items are fulfilled, the status will change to “Fulfilled.”