NEW YORK – American Express is facing off against the Justice Department this week in a court battle that could shape the future of the credit card industry. The suit, which concerns the fees merchants pay every time a customer uses plastic, is the culmination of a four-year war between federal authorities and the New York-based credit card giant.

The outcome won’t just affect the way American Express does business, but will likely impact merchants and consumers at the checkout counter as well, according to a report from Time magazine.

Currently at stake is AmEx’s “take it or leave it” policy. Every time a customer pays with a credit card, the merchant must pay a processing fee, generally between 2% and 3% of the total purchase. American Express — which, according to the government, charges the highest merchant fees of any card network — forbids its merchant partners from offering customers incentives to use cards that are cheaper for the vender to accept.

DOJ is arguing that the policy is anti-competitive because AmEx — which accounts for 26% of all money transacted through credit cards in the U.S. — is too important for most businesses to drop. It also claims that customers, even those who use a different card, end up paying for AmEx’s higher rates because merchants are forced to compensate by increasing prices. American Express, of course, disagrees, saying that the company is too small to have an anti-competitive effect on the market. It also argues these higher fees are necessary to provide merchants with services like fraud reduction and other programs.

This is the latest battle in a four-year-old war over credit-card company business practices. In 2010, the Justice Department filed a lawsuit against MasterCard, Visa and American Express for various merchant restrictions that the department found ultimately result in consumers paying more for their purchases. Visa and MasterCard quickly settled, later agreeing to a record-high $5.7 billion antitrust settlement with U.S. merchants over alleged fee fixing. But AmEx held out.

In 2013, it reached a separate peace with merchants, allowing them for the first time to add a surcharge to AmEx purchases as long as they added the same charge to all credit-card transactions — the “take it or leave it” policy. But the settlement failed to satisfy the Justice Department, which now seeks to force AmEx into the same deal it cut with Visa and MasterCard.

If American Express loses, merchants would be allowed to offer additional discounts to credit-card users with cards that charge lower fees. This won’t pacify those who say customers are paying the same prices as before plus new credit-card processing fees, but it does mean certain credit-card users might pay less than others.