Shares of United Parcel Service Inc. UPS, -0.05% dropped 2.3% and of FedEx Corp. FDX, -0.53% shed 2.0% in premarket trade Thursday, after Bloomberg reported that Amazon.com Inc. AMZN, -1.78% was testing a new delivery service. The report, which cited people familiar with the plan, said Amazon is looking to make more products available for free two-day delivery and to relieve overcrowding in its warehouses. The service, a project Amazon is calling Seller Flex, has begun on a trial basis in the West Coast and is expected to have a broader rollout in 2018, the report said. As part of the service, Amazon will oversee pickup of packages from warehouses of third-party merchants and delivery to customers, the report said, which is currently handled by UPS and FedEx. While those couriers will still be used, Amazon will decide how a package is sent rather than leave that to the merchant, the report said. Amazon's stock gained 0.5% in premarket trade. Year to date through Wednesday, shares of UPS has gained 3.8% and FedEx has run up 19%, while the Dow Jones Transportation Average DJT, -1.04% has tacked on 9.7% and the Dow Jones Industrial Average DJIA, -0.87% has advanced 14.7%.