Shares of Workhorse Group Inc. /zigman2/quotes/207314632/composite WKHS -4.76% fell nearly 50% at the end of the trading session Tuesday following news that Oshkosh Corp. /zigman2/quotes/200559252/composite OSK +0.75% won a U.S. Postal Service contract worth billions. Shares of Oshkosh were rising around 5% after a bigger spike on the news. The contract calls for Oshkosh Defense to help modernize the postal service's delivery vehicle fleet with fuel-efficient and hybrid vehicles. The contract calls for the delivery of between 50,000 and 165,000 vehicles over a period of 10 years, Oshkosh said.