The app does not control fulfillment or adjust fulfillment statuses in any way. Preorder items are marked with a line item property labeled “This is a preorder product”, but Shopify handles fulfillment logic.
If you want all items in an order, both in-stock and preorder, to ship together, you’ll need to manage that manually in your Shopify admin or through a third-party fulfillment app.
Configure shipping rules with your shipping or logistics provider. Request them to set up rules so that free shipping applies to all shipments within an order when the total order value exceeds your free-shipping threshold.