Flat Rate vs. Interchange Plus: Compare Your Real Cost

Flat-rate pricing charges one stated percentage for eligible transactions, while interchange-plus pricing passes through card-specific interchange and adds a processor markup and other disclosed fees. Neither model is automatically cheaper. Compare them by calculating the effective rate on the same statement, separating debit from credit, and adding monthly, per-transaction, equipment, and contract costs.

The two prices inside one processing quote

A card payment does not create one universal fee. The underlying interchange depends on the card and transaction, while the processor's price determines how those costs are packaged for the merchant. A flat-rate quote bundles eligible costs into a stated rate. An interchange-plus quote passes through the applicable interchange and then adds a stated processor markup, often with other line items.

That distinction matters because the headline can hide the denominator. “Interchange plus 0.25%” is not a 0.25% total cost. “2.79% flat” may be simple, but you still need to confirm which card brands, transaction types, and channels qualify. The only honest comparison puts both offers against the same sales volume and transaction mix.

Flat rate: predictable, but read the boundaries

Flat-rate pricing is easiest to forecast when the offer clearly states what is included. Multiply eligible card volume by the rate, then add any charges the quote lists separately. If every included transaction is priced the same way, the month-to-month percentage is stable even when the card mix changes.

The tradeoff is that a bundled rate can cost more than a low-cost mix would under a well-priced interchange-plus plan. It can also protect a merchant from having to decode dozens of interchange categories. Before accepting a flat quote, ask these questions in writing:

• Which card brands are included?
• Is the rate limited to card-present sales?
• Are keyed, online, manually entered, or premium-card transactions different?
• Are monthly, statement, gateway, batch, compliance, chargeback, or hardware fees separate?
• Does the rate change after an introductory period or volume threshold?

A flat rate is only flat inside its defined boundary. Transactions outside that boundary need their own line in the comparison.

Interchange plus: transparent, but variable

Interchange-plus pricing separates the card-specific pass-through cost from the processor's markup. A quote might express the markup as a percentage, a per-transaction amount, or both. Monthly and event-based fees may still sit outside that markup.

The Federal Reserve Bank of Kansas City's interchange resource, last updated in August 2026, shows why the pass-through portion can vary: published schedules differ by card type, merchant category, and merchant size. The bank also cautions that fee schedules do not necessarily equal what issuers ultimately earn and that some merchants may receive lower rates. In other words, a generic “interchange rate” copied from a sales deck is not your store's measured cost.

Interchange-plus can be attractive when the markup is competitive and the store's card mix produces lower pass-through costs. It can be harder to forecast because the total changes with the cards customers use, average ticket, acceptance channel, and the offer's fixed fees. Transparency helps only when the statement is readable and every fee is included in the calculation.

Keep regulated debit separate from credit

Do not use a debit statistic as a shortcut for credit-card cost. The Federal Reserve Board's Regulation II debit interchange page, last updated December 19, 2025, states that a covered issuer's fee for an eligible electronic debit transaction is capped at $0.21 plus 0.05% of the transaction value, with a possible $0.01 fraud-prevention adjustment. Exempt issuers and certain exempt transactions are treated differently.

The same page's 2024 data show different average debit interchange results for covered and exempt transactions. Those figures describe debit interchange, not the processor's full merchant price and not credit-card pricing. When reviewing a statement, keep at least three buckets: regulated debit, other debit, and credit. If American Express or card-not-present sales are priced separately, give them their own buckets too.

Calculate the effective rate from one statement

Use a complete, ordinary month rather than a promotional quote. Find the settled card sales volume and all processing-related fees for that same period. Then calculate:

Processing effective rate = total processing fees ÷ settled card sales × 100

Write down exactly what you included. Interchange, network charges, processor markup, per-transaction fees, monthly account fees, gateway fees, and other payment charges belong in the numerator when they appear. Keep POS software subscriptions and equipment purchases on separate lines first; add them later when comparing the all-in operating cost of each option.

Refunds and chargebacks can make statements use different volume definitions. Do not quietly switch denominators. Record the statement field you used and apply the same method to every offer. If the processor offers a rate match, confirm whether it matches the processing-only effective rate or a narrower subset.

A hypothetical worksheet shows why card mix matters

Consider a store with $30,000 in settled card sales and 600 transactions in one month. These are hypothetical numbers, not market averages or a quote.

Offer A: flat rate

• Flat rate: 2.79%
• Processing cost: $30,000 × 2.79% = $837
• Processing effective rate: $837 ÷ $30,000 = 2.79%

Offer B: interchange plus, lower-cost mix

• Interchange and network pass-through shown on the statement: $570
• Processor markup: 0.25% of $30,000 = $75
• Per-transaction markup: $0.10 × 600 = $60
• Monthly fee: $15
• Total processing cost: $720
• Processing effective rate: 2.40%

Now keep every Offer B term the same but change the hypothetical pass-through amount to $750 because the card and transaction mix differs. Total processing cost becomes $900, or 3.00%. The markup did not change; the underlying mix did. This is why one retailer's interchange-plus result cannot be copied into another retailer's forecast.

Run the worksheet with your statement, not these numbers. If sales are seasonal, test a slow month and a peak month. Fixed fees weigh more heavily when volume is low, while percentage differences produce larger dollar gaps when volume rises.

Convert the percentage gap into margin dollars

A rate difference feels small until it is applied to annual card volume. On $30,000 a month, a 0.30 percentage-point difference equals $90 monthly or $1,080 over twelve identical months. That is arithmetic, not a promise of savings; actual volume and mix will change.

Then connect the fee to the merchandise. The earlier guide on how card fees cut into retail margin explains why a fee taken from revenue consumes a larger share of the profit dollars left after product cost. Compare payment offers in dollars, then ask whether the operational differences justify the gap.

Compare the contract, workflow, and failure cases

Price is not the whole decision. Put these items next to the effective-rate calculation:

• Contract term and renewal language
• Early-termination or account-closure charges
• Funding schedule and weekend treatment
• Chargeback fee and response workflow
• Hardware ownership, lease terms, and replacement process
• Support hours and escalation path
• Card-present, keyed, online, and offline pricing
• Data export and processor-switching process

Ask the provider to mark “included,” “separate,” or “not offered” beside each item. A lower effective rate can lose its advantage if the setup does not fit the store's sales channels or locks the owner into equipment and terms that are costly to exit.

Apply the same test to VoVi's current pricing

After the neutral comparison, VoVi's pricing page provides two current lanes to test with the same worksheet. As checked on September 14, 2026, it states $0 per month with VoVi Payments at 2.79% flat for card-present Visa, Mastercard, and Discover, or a matched lower rate; American Express is 3.5%. The alternative is $99 per month per location while keeping the merchant's own processor.

Those are VoVi's published terms, not a claim that one lane is cheaper for every store. Use the statement calculation first. Compare the current processor's true effective rate plus the $99 software line against the published processing-funded lane, and confirm eligibility and final terms before switching.

The one-page decision sheet

Finish with one page that a partner or manager can audit:

• Statement month, settled volume, and transaction count
• Volume by debit, credit, American Express, and sales channel
• Every processing fee and the resulting effective rate
• Software, hardware, and contract costs shown separately
• A slow-month and peak-month scenario
• Written answers for exceptions, rate changes, and termination
• The offer date and person who confirmed the terms

The better pricing model is the one that produces the lower acceptable all-in cost for your actual mix, with terms your store can operate and exit. Recalculate after the first full statement. If the result differs from the quote, identify the exact card category or fee that moved before assuming the entire model was wrong.