What is it called when a fee is charged by brokers only when investors are buying and selling their assets?
From a HELPFUL person please!

Waterway company’s record of transactions for the month of april was as follows. purchases sales april 1 (balance on hand) 672 @ $6.00 april 3 560 @ $11.00 4 1,680 @ 6.08 9 1,568 @ 11.00 8 896 @ 6.41 11 672 @ 12.00 13 1,344 @ 6.51 23 1,344 @ 12.00 21 784 @ 6.61 27 1,008 @ 13.00 29 560 @ 6.79 5,152 5,936 (a) calculate average-cost per unit. (b) assuming that periodic inventory records are kept in units only, compute the inventory at april 30 using lifo and average-cost. (c) assuming that perpetual inventory records are kept in dollars, determine the inventory using (1) fifo and (2) lifo. (d) compute cost of goods sold assuming periodic inventory procedures and inventory priced at fifo.