FedEx (NYSE: FDX) on Wednesday said it intends to acquire e-commerce platform ShopRunner for an undisclosed price, adding a new dimension to the shipping giant's growing rivalry with Amazon.com (NASDAQ: AMZN). ShopRunner is a membership program that offers free two-day shipping, free returns, and other benefits when members buy from one of its 100 retail partners. The benefits are similar to what Amazon offers through its Prime service.