In a challenge to Amazon.com Prime, Walmart said on Wednesday that it will offer a 30-day trial of ShippingPass, its two-day, unlimited shipping service. "Starting today, we're offering a free 30-day trial of ShippingPass, which gives you unlimited two-day shipping," said Fernando Madeira, CEO of Walmart.com U.S., in a release. "If you already have ShippingPass, there's also good news -- we're going to give you an extra month for free.