CHICAGO - Wal-Mart Stores Inc is planning to test a new unlimited online shipping service this summer for $50 per year, a move that may hurt Amazon’s annual $99 Prime shipping service. Wal-Mart’s service, which will be by invitation only for now, will offer selected products on the company’s website to customers within three days or less, company spokesman Ravi Jariwala told Reuters. Wal-Mart offers more than 7 million products on its website. “Depending on customer feedback we will see how the program evolves,” Jariwala said. The move underscores Wal-Mart’s efforts to scale its online business rapidly and gain a share of a market dominated by Amazon. Amazon’s launched its $99-a-year shipping service called n Prime, a decade ago with the guarantee of standard, reliable two-day shipping on online orders. Since then Prime has become the cornerstone of Amazon’s growth and the e-commerce company said U.S. Prime memberships grew 50 percent last year.