Debit or credit: why the size of the sale changes your real cost

One customer pays $18 with an Interac Debit card. The next pays $180 with a credit card. At the counter these two payments look the same, but they cost your business different amounts to process. The gap has nothing to do with the sale amount itself. It comes from how each network calculates its fees in the first place.

Two different pricing logics

Debit and credit aren't two versions of the same system with different numbers attached. They run on two separate pricing logics. For Interac Debit, interchange, the share your business sends to the customer's bank, is set at zero. The only charge Interac Corp. applies to a chip-and-PIN or contactless transaction is a flat network fee, set on a cost-recovery basis. That fee doesn't change whether the sale is $5 or $500.

A credit card transaction works differently. The network, Visa, Mastercard or another, sets an interchange rate that applies as a percentage of the sale amount. The higher the sale, the more dollars that percentage pulls out, even though the percentage itself stays the same.

There's also a small wrinkle depending on how the debit card is presented. Inserting the chip carries no interchange at all. Tapping the card for a contactless payment triggers a separate network fee, but it is still flat rather than proportional. Either way, the calculation doesn't move with the size of the sale.

What makes one credit card's rate different from another's

A credit card's interchange rate isn't fixed either. It depends on several things the merchant has no control over at the moment of sale:

• The card brand: Visa, Mastercard or another
• The type of card used: a plain card with no rewards program typically carries a lower rate than a premium card offering points, cash back or travel perks
• How the transaction happens: an in-person sale, where the merchant actually sees the card, is treated differently from a sale taken online or over the phone

That last distinction exists because a transaction where nobody sees the physical card carries more fraud risk for the issuing bank. Visa and Mastercard review these rates periodically, usually twice a year.

The effect on a small sale versus a large one

Here's what that structural difference means in practice. On a small sale, a coffee or a quick errand, the gap between the two payment methods stays modest in dollar terms, even though the underlying math is different. On a bigger sale, a vehicle repair or a piece of furniture, the gap becomes much more visible, because credit fees grow with the sale amount while debit fees stay roughly flat.

That's why a business with a high average ticket, a garage or a furniture shop, for instance, feels the difference between the two payment methods far more than one where almost every sale is under $20, like a café or a corner store.

Flat-rate pricing versus pass-through pricing

This debit-versus-credit distinction either shows up clearly on your monthly statement or gets buried in it, depending on how your statement is built. Some merchants pay a single flat rate that blends every card together: debit, standard credit and premium credit are all billed at the same percentage, regardless of what each transaction actually costs the processor. Others pay based on the real interchange for each transaction, plus a fixed markup for the processor.

Under a blended rate, a debit sale can end up costing you more than it should, because it's absorbing part of the cost of premium credit cards. Under a rate tied to real interchange, the gap between debit and credit shows up clearly, transaction by transaction, but your bill swings more from month to month depending on the mix of cards your customers use.

Neither approach is wrong on its own. What matters is knowing which one applies to your business, because it completely changes what a customer choosing debit over credit is actually worth to you.

What you can check on your own

You don't need to memorize rates to understand where your money goes. Start by looking at whether your sales volume is mostly debit, standard credit or premium credit: that determines your overall cost far more than which processor's name is on your statement. Then, next time you talk to your payment processor, simply ask how they bill each card category: on real interchange, which varies transaction by transaction, or on a flat rate that smooths that variation out.

The answer tells you whether your bill reflects what's actually happening at the counter, or an average that can work for you or against you depending on your customers' mix of cards.