Wal-Mart Stores willstart testing a new unlimited online shipping service this summer for $50 per year, which is signnificantly lower than Amazon's corresponding $99 Prime shipping service. Wal-Mart's service will offer selected products on the company's website to customers within three days or less. It will start as an invitation only service and its future will depend on customer feedback, according to Amazon.

Amazon's $99-a-year Prime shipping service promises reliable two-day shipping on online orders.