Linnworks records cost of stock shipped with the order at the point of dispatch (at the point of processing the order). This figure allows you to derive the profit margin you are making for each product, providing your stock value is accurately recorded in the system. 

The stock unit cost is taken from the current stock value at the point of processing the order and saved against the order item. Dispatched Stock Cost in available as a value to select Pivotal Analytics in Product sales by month and Product sales by month (composites) pre-build reports, which in turn is derived from Product Sales cube.