Most people assume credit card companies make their money from interest — the 24% APR that kicks in when you carry a balance. And that's true for a lot of cardholders. But there's a second revenue stream that runs quietly in the background, hitting every single transaction, whether you're a responsible payer or a chronic debtor. It's called interchange, and it's one of the most elegant money machines in consumer finance.

Every time you tap your card at a coffee shop, three things happen almost simultaneously. The payment goes through. You get your coffee. And the coffee shop receives slightly less money than you paid. That gap — typically 1.5% to 3.5% of the transaction — is the interchange fee, and it flows to your card's issuing bank. Buy a $5 latte, and the shop might net $4.85. Buy a $2,000 laptop, and the retailer hands over $30 to $70 before a single customer walks out the door.

Visa and Mastercard set the interchange schedules, and they're not negotiating with individual coffee shops. The rates vary by card type (a rewards card charges more than a basic card), by merchant category (airlines and hotels pay different rates than grocery stores), and by whether the card was physically present. Merchants can theoretically refuse to accept cards — but in practice, going cash-only costs more in lost sales than the interchange fees do. So they pay. Every day, on every transaction, to every card network, for every customer who swipes.

This creates a quiet subsidy that most people never notice. Cardholders — especially those using premium rewards cards — get real benefits funded by interchange. But those fees get baked into the prices every retailer charges. The coffee shop that's losing 3% on every card transaction isn't absorbing that cost forever; it's folded into the $5 price of the latte. Cash customers pay it too, invisibly. They're subsidizing the miles you're earning on your travel card. The interchange machine runs on everyone, whether they're swiping or not.