If most of your customers pay with debit cards, flat rate processing can cost you more than interchange plus, because a flat rate charges one percentage on every card while interchange plus passes through what each card actually costs. Debit is where those two numbers can sit furthest apart. This guide shows you how to tell, from your own statements and in about an hour, whether you are one of the businesses paying that gap, and when a flat rate is still the sensible choice. We explain every term as we go, and we point to a pricing model published by NPS, with its limits stated plainly.
The short answer
- Debit is where flat rate and interchange plus differ most. A flat rate charges the same percentage on a debit card as on a premium rewards card. Interchange plus lets a cheaper card cost less.
- Not every debit card is cheap. The federal cap covers debit cards from large banks. Debit cards from smaller banks and credit unions can cost more, so “mostly debit” does not automatically mean “mostly capped debit”.
- You can check this yourself. Your effective rate, your debit share and your average sale are enough to see whether a flat rate is likely to be costing you more.
- Flat rate is still right for some businesses. Very small tickets, very low volume and brand-new businesses can be better off with the simpler plan.
- Ask for the numbers in writing on your own statements, not on an average business.
Why debit sales are where the two pricing models split
Every card sale carries a cost set by the card networks, called interchange. It is paid to the bank that issued your customer’s card. Interchange is not one number. It depends on the type of card, how the card was used and who issued it. A premium rewards credit card costs more to accept than a basic debit card, because the rewards have to be paid for somewhere.
A flat rate hides that spread. You pay one percentage plus a fixed amount, such as 2.9% plus 30 cents, on every card, whatever it cost to process. Interchange plus, also called pass-through or cost-plus pricing, works the other way. You pay the actual interchange and card network fees for each sale, plus a markup that is stated up front. Our guide to the four merchant account pricing models compares flat rate, tiered, interchange plus and membership pricing in more detail.
What debit actually costs
Capped debit costs far less than a flat rate
- Flat rate (e.g., Stripe/Square)2.9% + 30¢Every card, same rateSquare invoices: 3.3% + 30¢; keyed cards: 3.5% + 15¢
- Capped debit (interchange plus)0.05% + 22¢Large-bank debit card, in personExempt small-bank debit: 0.80% + 15¢ in person; 1.65% + 15¢ online
Debit matters most because of a federal rule. Since 2011, interchange on debit cards from the largest banks has been capped by a Federal Reserve rule called Regulation II, which came out of the Durbin Amendment. We explain that in our guide to what the Durbin Amendment changed for small merchants. On a capped debit card, the bank’s share is small. If you are on a flat rate, you still pay the full flat percentage on that card. If you are on interchange plus, the low cost shows up on your statement.
What the numbers look like in NPS’s pricing study
NPS published a 2026 pricing study, “The Flat-Rate Tax”, written by Allen Kopelman, our CEO. You can read it here. Please read what it is before you rely on it. It is a cost model, not a survey of real merchants. It prices the same mix of debit and credit sales under flat-rate pricing and under interchange pass-through, using published rate schedules. NPS sells pass-through pricing, so the study comes from a processor with an interest in the answer. Every input is listed in the study so that you can check it.
In the study’s model, the debit share of card sales falls as the ticket grows: 40% at a $15 average sale, 35% at $40, 30% at $75, 25% at $150, 22% at $300, 15% at $1,000 and 10% at $2,500. The study says those ranges come from NPS merchant experience and are used as model inputs. It also says the Federal Reserve’s own payments data shows debit cards remain the majority of U.S. card payments by count.
Model inputs by ticket size
Debit share falls as average ticket grows
| Average ticket | Debit share of card sales | Channel noted in study |
|---|---|---|
| $15 | 40% | In-person, small tickets |
| $40 | 35% | — |
| $75 | 30% | — |
| $150 | 25% | Online / invoice example |
| $300 | 22% | — |
| $1,000 | 15% | — |
| $2,500 | 10% | — |
With those inputs, the study reports that businesses selling online or by invoice pay 19% to 31% less in processing costs on pass-through than on flat rate, across average tickets from $15 to $2,500. It gives one worked example: a business collecting $50,000 a month through invoices at a $150 average ticket keeps about $6,100 a year by moving from flat rate to pass-through. Those are results in the model, and “up to” is the right way to read them. Your own result depends on your own statement.
The study also lists an assumption that matters for this question. It assumes 35% of debit volume comes from small-bank cards that are not capped. We come back to that below, because it is the main reason “mostly debit” is not the whole answer.
Step 1: find out how much of your volume is really debit
Most people guess their debit share, and the guess is usually off. Customers under 30 tend to use debit more. Young businesses often pay business-to-business invoices with a debit card because they do not have a company credit card yet. Big-ticket purchases lean toward credit. Those patterns come from NPS’s experience with the businesses we work with, not from a survey, so treat them as a reason to check rather than as a fact about your customers.
Look at your last three monthly statements. Some statements split sales by card type, such as debit, credit and rewards. If yours does not, that is worth a question to your provider on its own. Add up debit sales and divide by total card sales. Do it for three months, because one month can mislead you.
Check your own numbers
Four steps to see if flat rate costs you more
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Find your debit shareLook at your last three monthly statements.
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Calculate your effective rateDivide your total processing fees by your total card sales for the same month.
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Check your average sale and how customers payThe gap widens as the ticket grows, and the study found it largest for online and invoice payments.
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Remember not all debit is cappedDebit cards issued by smaller banks and credit unions are exempt, so they can carry higher interchange.
If your statement does not show debit separately, ask your provider for a report that does. A provider that prices every card the same may not break it out, which is itself a sign of how the plan works. Our guide on how to read your merchant statement walks through the sections you will see.
Step 2: calculate your effective rate
Your effective rate is your total processing fees divided by your total card sales for the same month. If a business sold $20,000 on cards and paid $500 in total fees, its effective rate is 2.5% (an illustration only, so use your own figures).
NPS’s statement guide says an effective rate above 2.5% may mean you are paying too much, and that average fees typically fall between 2.0% and 2.7% for retail businesses that sell to consumers, depending on the mix of debit and credit. It also says in-person PIN debit tends to cost less, and that online sales and keyed-in sales run a little higher. If you take mostly debit in person and your effective rate sits near the top of that range, that is a reason to dig further. Our guide on why your effective rate is higher than the rate you were quoted lists the seven usual reasons for a gap.
Compare the result with the rate you were quoted. The point of this step is to learn whether you are in the normal range for your kind of business, not to prove anything on its own.
Step 3: look at your average sale and how customers pay
Two things change the answer more than almost anything else: the size of your average sale and whether customers pay in person, online or by invoice.
A fixed per-transaction fee takes a bigger share of a small sale. The study says in-person businesses with very small tickets, around $15 and under, save little by switching, because per-transaction fees take up a larger share of each sale. The gap widens as the ticket grows, and the study found it largest for online and invoice payments.
Invoices deserve a second look. The study notes that flat-rate providers often add fees on top of the headline rate. It quotes Stripe’s published U.S. pricing, accessed October 2026: an Invoicing fee that adds 0.4% per paid invoice (Stripe’s own page, read November 13, 2026, lists 0.4%), an extra 0.5% for manually entered cards, and a $15 fee per dispute. It also quotes Square’s published U.S. pricing, accessed October 2026: 3.3% plus 30 cents for invoices, compared with 2.6% plus 15 cents for in-person payments, and 3.5% plus 15 cents for keyed or saved cards. Prices change, so read each provider’s current pricing page before you rely on those lines. We cover the Stripe and Square sides of this in how much you are really paying on Stripe, Square, PayPal or QuickBooks.
Step 4: remember that not all debit is capped
This is the step most articles skip. The federal cap applies to debit cards issued by large banks. Debit cards issued by smaller banks and credit unions are exempt, so they can carry higher interchange. Two customers can hand you two debit cards on the same afternoon and the cost to you can differ because of the issuer, not because of anything you did.
For that reason we do not say “most debit cards cost almost nothing to accept”. In its model, the NPS study assumes 35% of debit volume is exempt, and it prices that exempt debit higher than the capped kind: 0.80% plus 15 cents in person, and 1.65% plus 15 cents online and by invoice, against 0.05% plus 22 cents for capped debit. That is still well under a flat rate such as 2.9% plus 30 cents, as the study uses, but it is far from free. We take this up in detail in our guide to whether the debit cap applies to small-bank debit cards.
The practical meaning for you is that your debit share is only half the picture. The other half is who issued the cards. Your provider may be able to report that, and a transparent interchange-plus statement will often show it in the lines.
When flat rate is still the better deal
Flat rate is not a trick, and it is sometimes the right call. NPS describes flat rate as a “starter” model that suits low-volume and brand-new businesses, and the study says so too.
- Very low volume. If you run a few thousand dollars a month, the time you would spend changing providers may be worth more than the saving.
- Tiny tickets. At around $15 per sale or less, fixed per-transaction costs take a large share on any plan.
- You want simple. One number, quick sign-up and no statement to read have real value for a new business.
- Your cards skew expensive. The study notes business and commercial cards can cost more, which narrows the gap. A business paid mostly with company credit cards has less to gain.
NPS describes flat rate as a fit for startups processing under $10,000 a month and interchange plus as a fit for businesses processing over $25,000 a month. Treat those figures as rules of thumb, not a promise. We compare the two in flat-rate versus cost-plus processing.
What interchange plus asks of you in return
Interchange plus is not free of trade-offs. The statement is longer, and the monthly cost moves with your card mix, so it is less predictable than one flat percentage. Some providers add monthly fees, gateway fees or minimums that a flat-rate plan does not have. That is why the comparison must use total cost, not the markup alone. Our guide on what transparent merchant pricing means lists the fee lines to ask about.
It also matters how a quote is built. Ask for the markup in writing, whether it is fixed or can change, and whether debit is shown on its own line. If a provider will not show you the debit line, you cannot tell whether the cap is reaching you.
Questions to ask before you change anything
- What share of my card sales is debit, and can you show it on my statement?
- Of that debit, how much comes from large-bank cards and how much from smaller issuers?
- What is my total monthly cost, including every fixed fee, on my last three statements?
- What would the same three months have cost on interchange plus, with every fee listed?
- Does the quote include extra fees for invoices, keyed-in cards or disputes?
- Is the markup fixed, and what can change it?
- What happens if the Federal Reserve changes the debit cap? (The study notes a proposed change remains unresolved.)
Before you switch
Questions to ask any processor in writing
- What share of my card sales is debit, and can you show it on my statement?
- Of that debit, how much comes from large-bank cards and how much from smaller issuers?
- What is my total monthly cost, including every fixed fee, on my last three statements?
- What would the same three months have cost on interchange plus, with every fee listed?
- Does the quote include extra fees for invoices, keyed-in cards or disputes?
- Is the markup fixed, and what can change it?
The best proof is a side-by-side comparison on your own statements. NPS offers a free statement review, and you can ask for one without committing to anything. For the other direction, our guide on where Stripe fees actually go explains what sits behind a flat rate.
For the full picture across every sales channel and average ticket, read our guide to cheapest payment processing in 2026.
Who wrote this, and where NPS fits
Nationwide Payment Systems publishes this guide. We are a payments company based in Fort Lauderdale, Florida, in business since 2001, and we sell interchange-plus processing, so we are not neutral. We do not publish a rate, because the right price depends on your volume, ticket size, card mix and industry. The pricing study we cite was written by our CEO and is a model, not a survey. Please check any provider’s current pricing page, since flat-rate prices change. You can browse our payments guides for more on fees and pricing.
We are not the right fit for everyone. If you are new, run very low volume, or want the simplest possible sign-up, a flat-rate plan may suit you better. Ask any provider for pricing in writing, based on your own statements.
Bottom line
If most of your sales are debit, flat rate can cost you more than interchange plus, because it charges the same percentage on a cheap card as on an expensive one. How much more depends on your ticket size, whether customers pay in person, online or by invoice, and how much of your debit comes from capped large-bank cards. Check your debit share and effective rate across three statements, then ask for a side-by-side on your own numbers. If you would like a second opinion, contact our team, and if flat rate fits you better, we will say so.
Frequently asked questions
Is flat rate processing more expensive if I take mostly debit?
Often, yes. A flat rate charges one percentage on every card, so it does not pass on the lower cost of a capped debit card. In NPS’s 2026 pricing study, a model built on published rate schedules by a processor that sells pass-through pricing, online and invoice businesses paid 19% to 31% less on pass-through. Your result depends on your own statement.
How do I find out how much of my volume is debit?
Check your last three monthly statements for a breakdown by card type. If it is not there, ask your provider for a report that separates debit from credit. Divide debit sales by total card sales for each month.
Are all debit cards cheap to accept?
No. The federal cap covers debit cards from large banks. Debit cards from smaller banks and credit unions are exempt and can cost more. In its model the NPS study assumes 35% of debit volume is exempt.
Is there a business size where flat rate makes sense?
Yes. Brand-new businesses and very low-volume businesses often do well with the simplicity of a flat rate. The study also found that in-person businesses with tickets around $15 and under save little by switching.
Does interchange plus always cost less?
No. It can cost more if the markup or the monthly fees are high, or if most of your cards are business credit cards with higher interchange. Compare total cost on your own statements, not the markup alone.
What is interchange plus in plain words?
Interchange plus, also called pass-through or cost-plus, means you pay the actual card costs set by the networks and issuing banks for each sale, plus a stated markup. A debit card costs less than a rewards credit card, and you can see both on your statement.
Can I switch processors without stopping sales?
Often yes, but contracts differ. Ask about early termination fees, equipment leases and how long the changeover takes before you sign anything.
