Periodic Inventory System and Inventory Costing Methods In chronological order, the inventory, purchases, and sales of a single product for a recent month are as follows. 1. Using the periodic inventory system, compute the cost of ending inventory, cost of goods sold, and gross margin. Use the average-cost, FIFO, and LIFO inventory costing methods. (Round unit costs to cents and totals to dollars.) FIFO LIFO AVG. COST