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Rick Bowmer, File, Associated Press
In this Dec. 7, 2011 file photo, a shopper leaves a Costco store in Portland, Ore. Costco shoppers who have been limited for years to American Express credit cards may be able to pluck a new option from their wallets or purses next year when an exclusivity deal between the two companies expires on March 31, 2016.

Costco shoppers who have been limited for years to American Express credit cards may be able to pluck a new option from their wallets or purses next year when an exclusivity deal between the two companies expires.

American Express said Thursday that it hasn't been able to come to an agreement with Costco on renewal terms. The current agreement covers U.S. stores and dates back 16 years. It will end March 31, 2016.

The end of the partnership with Costco will negatively affect earnings and revenue growth in 2015 and 2015, the company said. Company shares tumbled 6 percent.

Costco is the world's second-largest retailer by revenue, according to the National Retail Federation.

The company posted revenue of about $8 billion last month alone.

"This relationship accounted for (8 percent) of the billed business, (20 percent) of worldwide loans, and (10 percent) of cards in force so it represents a large loss in our opinion," analysts with Jefferies and Co. wrote Thursday.

American Express said that Costco pays a significantly lower rate than the overall average it imposes on other retailers as part of the agreement. That arrangement gave the credit card company exclusivity for almost two decades, though debit cards and cash are also accepted.

American Express ended a similar agreement with Costco in Canada last year, a spokeswoman said.

Costco Wholesale Corp. did not immediately return a call from The Associated Press on Thursday. The company has 474 locations in the United States and 88 in Canada.

The expiration could clear the way for rivals Visa Inc. or MasterCard Inc. Shares of both companies rose Thursday.

American Express will attempt to increase business elsewhere to counter the loss of Costco and said it remains confident that it will be able to achieve its goal of earnings-per-share growth between 12 percent and 15 percent "over the moderate to long-term."

Because of the exclusivity that American Express was granted at Costco, it gave up the fatter revenue and profits it commands elsewhere. As a result, there will likely be a smaller-than-normal decline in the overall reported discount rate in coming periods, according to Jefferies.

But they believe the end of the partnership will hurt for a while.

"We are encouraged by recent investments and strategic initiatives from the company, yet we note that these are likely to take some time to make a considerable impact on earnings growth."

Share of American Express Co. fell $5.25, to $80.76 while broader indexes rose slightly. Costco shares climbed 8 cents to $147.54.