Veeqo will allow you to accept pre-orders from Shopify for products that you currently have no physical stock for in your warehouse. That means your real stock of 0 is set correctly in Veeqo, but the item will still be available for purchase on your Shopify.
Your pre-orders will come into Veeqo as Waiting For Stock and therefore sit separately from your Ready To Ship orders, to avoid any confusion.
How to enable pre-orders on Shopify?
1. Log into your Shopify Admin;
2. Go to Products>Inventory;
3. Find the product you want to accept pre-orders for and scroll to the Inventory section;
4. Set a checkmark for the options Track quantity and Continue selling when out of stock;
5. Click Save.
Orders for these pre-order products will go into Waiting For Stock and once you receive stock, you can simply allocate stock to these orders to make them Ready to ship.
Shopify has a great guide that makes it clear on your store that this product is on a backorder basis.