Airlines charge their customers separately for additional services; the rate is assessed by pound, with a minimum charge for small shipments. In the past few years, airlines have struggled to increase their rates. Despite strong demand for air transportation, capacity exceeds demand on most routes, making it a buyer's market except for a few routes and during peak periods. To optimize revenues, cargo airlines differentiate their services and products. Almost all cargo carriers offer an express shipment service that comma with special features. One of the most noteworthy features allows the customer to tender express shipments right up until the departure time, and the express freight is the first to be available for pick-up at the destination. Express services often come with a performance guarantee, possibly, including a refund in ease of a controllable delay. Other special services are o marketed for special handling needs, i.e., goods requiring animals, dangerous goods, and high-value item.