Product Cost = Cost of inventory on hand, also called inventoriable cost . They are assets until the products are sold. Once they are sold, they become expenses.
Thus, via application of the aforementioned definition, insurance on stock is probably a period cost as it is an administrative expense which is not usually apportioned / pro-rata'ed into the unitized cost basis of inventory.
The question of cost-assignment is one which concerns the extent of relationship between the incurrence and the cost object. In the case of inventory liability insurance, the degree of correlation between the object (stock) and the subject (insurance) is not sufficiently intact to justify attribution.