Shopping carts are seen outside a new Wal-Mart Express store in Chicago July 26, 2011. REUTERS/John Gress/Files

(Reuters) - Wal-Mart Stores Inc on Wednesday launched a free 30-day trial of ShippingPass, its two-day shipping program for shoppers in the United States, as the world’s largest retailer looks to take on Amazon.com Inc’s Prime subscription service.

Wal-Mart said that existing ShippingPass subscribers will get one month free. (bit.ly/292sPcG)

While Amazon Prime also offers two-day delivery on certain items, subscription to the service costs $99 a year. Wal-Mart’s two-day service costs $49 a year.

As with Prime, Wal-Mart’s service has no minimum order but it is valid only on items that are flagged with the ShippingPass logo. Wal-Mart does not offer same-day delivery, unlike Amazon which offers that option on a minimum order of $35.

Wal-Mart has announced a number of programs for its online business this year, including a partnership with ride-hailing services Uber and Lyft for grocery deliveries.

The innovations come at a time when Wal-Mart’s online sales growth is slowing. Sales through the company’s website and mobile app increased 7 percent in the latest quarter compared with 17 percent a year earlier.