Discover Financial Services coded five million consumer credit cards as commercial starting in 2007, and no one caught it for seventeen years. In April 2025, the FDIC ordered Discover Bank to return at least $1.225 billion to the merchants who paid the wrong rate. The root was not fraud. It was a four-digit code picked at onboarding that no one ever audited.

That code is called a merchant category code. Your processor chose it the day you signed your merchant agreement. It still sits on your account, setting the interchange rate on every card sale you run.

Most operators who accept cards have heard some version of this advice: let your processor handle the back office. They know the codes. You run the shop. It sounds right. It is not.

The mechanics are plain. Every card that runs through a merchant terminal hits an interchange rate. That rate is set by the card network, and it hinges on two things: the type of merchant and the type of card. The merchant's side is the MCC, a four-digit code from a standard list called ISO 18245. The card's side is the product type: consumer, commercial, rewards, premium.

Discover coded consumer cards as commercial. Commercial cards carry higher interchange. Visa's published schedule shows the gap. A consumer credit card runs interchange at 1.51% plus ten cents per transaction. A commercial credit card, the category Discover assigned to those five million consumer accounts, runs at 2.50% plus ten cents. That is a gap of nearly one full percentage point on every swipe.

On a $500 sale, the merchant paid roughly five dollars more than they should have. Five million cards. All of them wrong.

The Federal Reserve consent order states it in plain terms: "senior executives of the Firm were aware that numerous Discover consumer credit cards were improperly classified" and "failed to take adequate steps to correct" the problem.

The people running the system knew. They did not fix it.

Discover made the news because the dollar figure was large. But the flaw it exposed is not about one company. It is about how the whole system is built.

A PYMNTS analysis found that 15 to 20 percent of small business accounts are misclassified at onboarding. Not one in a hundred. One in five, at the low end. The processor picks the MCC from a dropdown menu. The merchant signs the agreement. No one checks the code again.

Acquirers do not review MCC accuracy after the first day. They do not flag it when your business shifts. The code sits in a field in a database, and every transaction you run prices off of it.

The problem is not that processors lie. The problem is that no one audits a code after it is set. Most people treat this as a billing flaw. It is a system flaw.

Your MCC was a choice someone else made on your behalf. It may be costing you on every sale. Once that is clear, three moves follow from it.

Pull your most recent processing statement. Look for a four-digit number labeled MCC, merchant category code, or SIC code. It is on the statement. Most operators pay that bill every month and never read past the total.

That is the part worth sitting with. You have been paying a fee set by a code you did not pick, printed on a document you did not read, for every year you have held this account.

Compare your MCC to the ISO 18245 list. The list is free and public. If your code does not match what you sell, you are in the wrong pricing tier. A consulting firm coded as a retail store pays a different rate. A service business coded as a restaurant pays a different rate. The code should match the work.

Write one letter to your acquiring bank or payment processor. Ask for a classification review. This is a one-time fix. It costs nothing to request. If the code is wrong, the rate shifts in your favor. If the code is right, you lose nothing but the time it took to check.

Running this once does something the old advice never did.

You see whether your rate tier matches your actual business type. You learn whether your interchange cost has been higher than it needed to be. You find out if your processor set your code from a dropdown and never looked at it again. And you get a clean read on a cost line that most operators have never questioned.

At the end of the review, ask three things.

→ Did the MCC on your statement match what you actually sell?

→ How long has that code been on your account without a single review?

→ What is the per-transaction cost gap between your current code and the correct one?

That is the gap between advice that sounds right and a system that proves itself.

Someone picked a code when you opened your merchant account. You signed the form. That code prices every card sale you run, and the FDIC just ordered $1.225 billion returned over the same kind of code, set the same way.

You either know what your code is, or you are trusting the same system that got it wrong five million times.