You can avoid syncing out of stock items to ShipStation by changing the sync mode to Fulfillments instead of Orders. Click here to find out how. Once set to Fulfillments, we will only sync the Packed items in the Shipment of the Sales Order, rather than all the items on the order. This way, you can only create a Shipment for items in stock, which will then sync over to ShipStation. You can automate the process of creating Fulfillments and packing items that are available using Commerce Automation. Click here to find out how.