Visa appealed to the public as it responds to Walmart's decision to stop accepting the network's cards in Canada, accusing the retailer of "unfairly dragging" millions of shoppers into private negotiations over card fees.

Walmart's Canada unit said Saturday it will reject Visa cards starting July 18 at certain stores in Ontario, eventually extending the ban to stores across the country. The subsidiary, which pays more than $100 million in credit-card fees annually, said it's looking out for shoppers and seeking to save them money.

"Walmart has initiated a public flight -- something we never wanted -- as they are using their own customers as negotiating leverage," Visa shot back Thursday in an open letter in the nation's newspapers. "Walmart is unfairly dragging millions of Canadian consumers into the middle of a business disagreement that can and should be resolved between our companies."

Canada is emerging as an alternate battleground for big retailers and card networks that have sparred over fees for decades in boardrooms, courtrooms and U.S. congressional hearings. In 2014, Costco Wholesale Corp. announced its Canadian stores were ending an exclusive deal with American Express Co., shifting customers to MasterCard Inc.'s network. Unable to reach agreement, Costco and AmEx ultimately decided to sever ties in their home U.S. market as well, with Visa taking over as the network of choice.

Visa and MasterCard already agreed in 2014 to lower costs for merchants in Canada amid government pressure. The two networks said they would reduce fees on consumer credit cards to an average effective rate of 1.5 percent for five years. As part of that effort, Visa and Walmart have been negotiating for several months, trying to work out a "fair and equitable" fee structure, the San Francisco-based payments network wrote in its letter.

"Walmart has always been treated as one of our most important global partners and we have always recognized their scale," Visa said in the letter. "We offered Walmart one of the lowest rates of any merchant in Canada. But Walmart is still demanding more."

Randy Hargrove, a Walmart spokesman, said "Visa's fees should be lower for everyone, whether they are a large retailer, small retailer or a charity."

"Canadians deserve better than paying a hidden fee that is 4-times higher than consumers pay" in some other countries, Hargrove said in a statement. "We are taking a stand for our customers because Visa's high fees can result in increased prices."

Walmart has been fighting for decades to reduce card fees that can eat into profits. It was among merchants that sued Visa and MasterCard in 1996, claiming they used their market dominance to force retailers to accept their debit cards, which carried higher fees. While that case was settled in 2003 with payments and policy changes, additional litigation followed alongside lobbying battles on Capitol Hill.

It's not the only big retailer putting pressure on card networks. Earlier this week, Home Depot Inc. sued Visa and MasterCard at an Atlanta-based federal court, accusing them of reaping high fees while refusing to require the use of PINs with chip cards to thwart fraud. Merchants can't refuse MasterCard and Visa-branded cards without losing customers and sales, according to the antitrust claim, which seeks unspecified damages.

"Regardless of how the cardholder's identity is confirmed, the chip makes data much more secure, rendering it almost useless to create fraudulent cards or transactions," a MasterCard spokesman said in an e-mailed statement. Visa said it will seek to consolidate the suit with litigation filed previously.

Credit-card networks began pushing chip cards years ago as a way to protect against counterfeits. Merchants were required to join banks and payment processors by Oct. 1 in embracing the technology or face liability for some fraudulent charges that occur in their stores. As retailers have gotten a feel for what it's like to shoulder the fraud costs, many have begun to deploy the technology.

To ease the transition, Visa said Thursday it will modify its liability policy to limit the number of fraudulent transactions issuers can charge back to merchants. Visa will block all counterfeit fraud chargebacks under $25 in the U.S. beginning July 22. Issuers also won't be able to charge back more than 10 fraudulent transactions per account starting in October.

"Merchants can expect to see 40 percent fewer counterfeit chargebacks, and a 15 percent reduction in U.S. counterfeit fraud dollars being charged back," Visa said in a statement. "A migration of this size takes time, and, hence, many merchants still require help to cross the finish line."

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