ShopRunner, a free shipping program for online consumers, said today that it had signed up 28 retailers, up from 15 when the service launched in October. ShopRunner is backed by e-commerce vendor GSI Commerce Inc. and aims to compete with Amazon.com’s Prime program.

The newest ShopRunner retailers include Borders, 194 in the Internet Retailer Top 500 Guide; Dick’s Sporting Goods, No. 130; PetSmart, No. 281; Calvin Klein;  MLB.com, No. 179; Eastern Mountain Sports, No. 422; and Igourmet.com.

ShopRunner says AutoZone Inc., No. 109, will join the program next month, with such online retailers as Ritz Camera (146), Musician’s Friend (38) and BoatersWorld.com set to join in early 2011.

“By continuing to expand our network of participating retailers, and increasing the breadth and depth of ShopRunner-eligible products that our members can get shipped fast and free, we aim to consistently enhance our member experience,” says Mike Golden, president of ShopRunner.

Consumers who pay $79 per year receive free two-day shipping and free returns from participating merchants. Consumers can click on the free shipping offer at checkout on the sites of participating e-retailers to learn more about ShopRunner and join the program.