Hello, it seems my equations are getting mixed up. Could you help me calculate the steps to this problem?

Hello, it seems my equations are getting mixed up. Could you help me calculate the steps to this problem?

As a manager of an operation's unit, your department uses 2,000 boxes of 2" screws each year. The supplier quotes you a price of $20 per box for an order size of 499 boxes or less, a price of $19 per box for orders of 500 to 999 boxes, and a price of $18 per box for an order of 1,000 or more boxes. You assign a holding cost of 20 percent of the price of the unit to this inventory.