Starting in about a month, customers will have 90 days to return items such as televisions, cameras and digital music players for a full refund instead of unlimited time, said Jeff Elliott, the Issaquah, Wash.-based company's finance director.

Costco was the only retailer with a no-strings policy, Elliott said. As prices for products such as flat-panel TVs fell amid competition from retailers including Best Buy Co. and Circuit City Stores Inc., profit margins were pressured as customers returned items and bought new ones.

"Some of our customers have been essentially trading up," Elliott said.

The previous policy cost the company about 8 cents a share in profit last year, Charles Grom, an analyst at J.P. Morgan Securities Inc., wrote today in a report about the new rules. He rates shares "overweight."

Shares of Costco, which has about 500 stores worldwide, fell 7 cents to $57.40 at 4 p.m. in Nasdaq Stock Market composite trading. They have gained 11 percent in the past year.

The return policy went into effect today at the company's 109 California stores.

Costco will apply the new policy nationwide in early April, Elliott said. The company's Web site has a March 26 deadline for online orders.

Costco will also roll out its concierge service to all U.S. stores at that time, Elliott said. The free service, which the company introduced in California last year, has cut down on returns, he said. Customers can call the service with questions about product set-up.