Boston

WAL-MART Stores will offer a US$50-a-year unlimited free-shipping service for online customers starting this summer, seeking to challenge the US$99 Prime service offered by retail rival Amazon.com.

The service will be available by invitation only in select markets, said Ravi Jariwala, a spokesman for Bentonville, Arkansas-based Wal-Mart. More than one million items - including apparel, sporting goods, electronics and toys - will be available for delivery in three days or less, he said.