Once just convenient trash receptacles, shopping bags have more than ever become walking billboards for a store or brand.

Retailers pass out more than 500 million shopping bags a year, about a third of them during the last six weeks of the year, said Jonathan Canno, chief executive of Equitable Bag Co., the nation’s largest supplier of shopping bags.

This year manufacturers are joining retailers in branding bags with logos, said Kimberly Rendleman, a principal at the design firm Lippincott & Margulies. Brands such as Coca-Cola, Eastman Kodak, Miller Beer and 7-Up are among those appearing on shopping bags, she said. Perfume makers are also big bag users.