If you have automatic fulfillment enabled, the PreOrder Now app can't prevent an order containing preorder items from reaching your shipping and fulfillment app.
Preorders are treated as regular orders in Shopify, so there isn't any special handling of these orders. They will proceed to your shipping app just like any other order.
We understand this might pose an issue when orders contain both preorder and non-preorder items, so it's crucial to consider this scenario before moving forward.
You might need to disable the automatic fulfillment of orders. We recommend consulting Shopify's documentation on this topic to explore your options further: Order Fulfillment