Shares of several large freight carriers slumped Wednesday after Amazon announced it's opening up its trucking services to companies beyond its own network. The e-retailer said it will offer less-than-truckload shipping to all businesses, not just those who ship goods into its warehouses and fulfillment centers, and deliver to any destination in the U.S., as part of its nascent Amazon Supply Chain Services program. Less-than-truckload refers to a service where companies carry shipments from