Interchange-plus vs
flat rate: which one
actually costs less

Every processing quote you will ever see is built on the same three ingredients: interchange (the issuing bank's cut), assessments (the network's cut), and markup (your processor's margin). Flat rate and interchange-plus are not different costs — they are different ways of showing you the same costs. One bundles all three into a single tidy number. The other unbundles them so you can see exactly what you are paying for. Which one costs you less depends almost entirely on what's underneath.

How each model bundles the same three things

A flat-rate quote looks like "2.6% + 10¢ on everything." Behind that one number, the processor has wrapped interchange, assessments, and its own markup into a single figure — then set that figure high enough to stay profitable across every card type you might run, including the cheapest ones. You never see the split. You just see the blend.

An interchange-plus quote looks like "interchange + assessments + 0.25% + 8¢." The first two pieces are passed through at cost — the exact published network rate for each transaction — and the only thing the processor adds is a stated, fixed markup. The margin is named out loud, which is the entire point.

• Flat rate: one number, markup invisible, set to cover the processor's worst-case card mix.
• Interchange-plus: cost-plus, markup explicit, the same on a cheap debit swipe and an expensive rewards card.

Where flat rate quietly overcharges

Here is the mechanism most owners miss. Flat rate charges you the same percentage no matter what the underlying transaction actually costs. That feels fair until you remember that interchange is not flat — a regulated debit card can carry interchange a fraction of what a premium rewards credit card carries.

So on a debit-heavy mix — quick-service food, convenience, anything with a low average ticket and lots of regulated debit — the true cost of a transaction is low, but flat rate still charges the full blended rate. The gap between what the transaction cost and what you paid is pure markup, and on debit-heavy volume that markup can be enormous. You are subsidizing the processor's margin on every cheap card you run.

Flat rate is fairest to the processor on exactly the card types where you should be paying the least.

Interchange-plus removes that asymmetry. When a cheap debit card runs, you pay the cheap debit interchange plus your small fixed markup — and you keep the savings instead of handing them over inside a blended number.

When flat rate is genuinely fine

None of this makes flat rate a scam. For some merchants it is the right call:

• Tiny or brand-new merchants. If you are processing a few thousand dollars a month, the dollar difference between models is small, and the simplicity of one predictable number has real value.
• Low, unpredictable volume. When you cannot forecast your card mix and statements would only confuse you, flat rate's "what you see is what you pay" clarity can be worth a few basis points.
• You value zero statement math over squeezing every point. Some founders would rather pay a small premium than ever reconcile an interchange line again. That is a legitimate trade.

The crossover usually arrives as volume grows. Once you are running meaningful monthly volume — especially with a debit-heavy or rewards-heavy mix — the invisible markup inside flat rate commonly outgrows the markup you would pay on a transparent interchange-plus deal, and interchange-plus starts winning on real dollars.

How to compare quotes apples-to-apples

The hard part is that a flat-rate number and an interchange-plus number are not directly comparable on their face — one includes the network costs and one doesn't. To compare honestly, convert both into the same shape: effective rate, meaning total fees divided by total card volume.

• Pull a real statement and compute your actual effective rate — every fee, divided by every dollar of card volume. That is your true all-in cost today.
• Ask any interchange-plus quote for its markup only — the "+ 0.25% + 8¢" part — and add it to your published interchange and assessments to model the all-in number.
• Make the processor model your mix, not a generic one. The same markup produces wildly different effective rates depending on your debit/credit split and average ticket.
• Watch for padding inside "pass-through." A few processors quote interchange-plus but inflate the interchange line. Cross-check a sample transaction against the published network rate.

If a processor will only quote you a single blended number and won't break out the markup, that refusal is itself information. The number you cannot decompose is the number you cannot negotiate.