Flat Rate vs Interchange Plus Pricing

Flat-rate and interchange-plus are the two most common pricing models in credit card processing, and choosing between them is the single biggest decision affecting your processing costs. The wrong choice can cost a mid-volume business $2,000–$5,000 per year in unnecessary fees.

How flat-rate pricing works

Flat-rate processors charge a single, blended percentage on every transaction regardless of the underlying card type. Stripe charges 2.9% + $0.30 for online transactions whether the customer pays with a basic debit card (interchange of 0.05% + $0.21) or a premium Visa Infinite rewards card (interchange of 2.40% + $0.10).

Major flat-rate processors and their current rates:

Processor	In-person	Online	Monthly fee
Stripe	2.7% + $0.05	2.9% + $0.30	$0
Square (Free)	2.6% + $0.15	3.3% + $0.30	$0
Square (Plus)	2.5% + $0.15	2.9% + $0.30	$49
Shopify Payments (Basic)	2.7% + $0.00	2.9% + $0.30	$39

The appeal of flat-rate pricing is simplicity. You know exactly what every transaction will cost before you process it. There are no surprises on your statement, no complicated interchange categories to decode, and usually no monthly fees (on the free tiers).

How interchange-plus pricing works

Interchange-plus (also called “cost-plus”) separates the three components of processing fees into distinct line items on your statement:

1. Interchange: The exact rate set by Visa/Mastercard for that specific card type and transaction method. This varies per transaction.
2. Assessment: The network fee (0.13%–0.14%). Usually passed through at cost.
3. Markup: The processor’s profit, stated as a percentage plus a per-transaction fee (e.g., + 0.40% + $0.08).

Major interchange-plus processors:

Processor	In-person markup	Online markup	Monthly fee
Helcim	IC + 0.40% + $0.08	IC + 0.50% + $0.25	$0
Payment Depot (Starter)	IC + 0% + $0.15	IC + 0% + $0.15	$49
Stax (Growth)	IC + 0% + $0.08	IC + 0% + $0.15	$99

The advantage of interchange-plus is that you pay the actual cost of each transaction plus a fixed, transparent markup. When a customer pays with a low-cost debit card, you pay less. When they use a premium rewards card, you pay more — but still less than you would on flat-rate because the markup is smaller than the flat-rate processor’s bundled margin.

Side-by-side cost comparison

To make this concrete, here are the monthly costs for an in-person business with $30,000/month in volume, 667 transactions (avg ticket $45), and a typical card mix of 40% debit, 35% basic credit, 20% rewards credit, 5% premium/corporate:

Component	Square (flat-rate)	Helcim (IC+)
Debit ($12,000, 267 txns)	$352 (2.6% + $0.15)	$131 (IC ~0.47% + markup)
Basic credit ($10,500, 233 txns)	$308 (2.6% + $0.15)	$239 (IC ~1.61% + markup)
Rewards credit ($6,000, 133 txns)	$176 (2.6% + $0.15)	$165 (IC ~2.20% + markup)
Premium/corporate ($1,500, 33 txns)	$44 (2.6% + $0.15)	$46 (IC ~2.60% + markup)
Monthly fee	$0	$0
Total monthly cost	$880	$581
Effective rate	2.93%	1.94%

The difference is $299/month or $3,588/year. The savings come overwhelmingly from debit transactions, where flat-rate processors charge 2.6% on a card that actually costs 0.47% in interchange — a markup of over 2 percentage points that is invisible on a flat-rate statement.

The break-even point

Flat-rate pricing is not always more expensive. At very low volumes, the simplicity and zero monthly fees make it the rational choice. Here is where the break-even typically falls:

In-person businesses

Monthly volume	Flat-rate cost	IC+ cost (Helcim)	Winner
$3,000	$88	$72	IC+ by $16/mo
$5,000	$147	$113	IC+ by $34/mo
$10,000	$293	$210	IC+ by $83/mo
$30,000	$880	$581	IC+ by $299/mo
$100,000	$2,933	$1,860	IC+ by $1,073/mo

For in-person businesses, interchange-plus is cheaper at almost every volume level because the debit card savings are so substantial. The only scenario where flat-rate wins is if your customers almost exclusively use premium credit cards (rare for most businesses).

Online/e-commerce businesses

The gap is narrower for e-commerce because online interchange rates are higher (2.0%+ vs 1.8% for in-person), and a smaller percentage of transactions are debit cards. Stripe’s 2.9% + $0.30 is closer to the actual cost of processing an online credit card transaction than Square’s 2.6% + $0.15 is for in-person.

Even so, interchange-plus typically saves 10%–20% for online businesses processing over $10,000/month, primarily because the markup (0.50% + $0.25 on Helcim) is lower than the effective markup embedded in Stripe’s 2.9% + $0.30.

Pros and cons at a glance

When flat-rate makes sense

Flat-rate pricing is the right choice in specific situations:

1. Very low volume (under $3,000/month). The savings from interchange-plus are too small to justify any added complexity. At $2,000/month, the difference might be $10–$15.
2. Seasonal or unpredictable volume. If you process $500 some months and $10,000 others, the zero monthly fee on flat-rate processors means you pay nothing during slow months. Subscription-model processors charge $99+/month regardless of volume.
3. Developer-first integrations. Stripe’s API, documentation, and developer tools are significantly more polished than most interchange-plus processors. If engineering time is expensive and integration complexity matters, the flat-rate premium can be worth it.
4. Almost entirely premium credit cards. If 80%+ of your transactions are high-interchange rewards or corporate cards (common for luxury retail or B2B), flat-rate processors absorb the higher interchange cost. On interchange-plus, you’d pay more for each of those transactions.

When interchange-plus makes sense

1. Processing over $5,000/month consistently. The savings scale linearly with volume. At $50,000/month, the difference can exceed $500/month ($6,000/year).
2. Significant debit card volume. Businesses where 30%+ of transactions are debit (grocery, gas stations, quick-service restaurants) save the most on interchange-plus because the debit interchange rate is 0.05% + $0.21 vs. the 2.6% flat rate.
3. Low average ticket size. Per-transaction fees matter more when tickets are small. Square’s $0.15 per transaction on a $10 sale is 1.5%. Helcim’s $0.08 per transaction on the same sale is 0.8%. On 3,000 transactions/month at $10 average, that difference alone saves $210/month.
4. You want negotiation leverage. Interchange-plus markups are negotiable. As your volume grows, you can demand lower markups or switch processors easily because the interchange component is identical across all processors.
5. Cost optimization is a priority. If you are actively managing expenses and willing to spend 15 minutes per quarter reviewing your statement, interchange-plus gives you the data to optimize.

The third option: subscription pricing

Subscription-model processors like Stax ($99–$199/month) and Payment Depot ($49–$199/month) take interchange-plus a step further by charging zero percentage markup. You pay interchange + a flat per-transaction fee + a monthly subscription.

This model becomes the cheapest option at higher volumes because the fixed monthly fee is spread across more transactions. At $30,000/month with 667 transactions, Stax’s total cost (interchange + $0.08/txn + $99/month) is approximately $706 — lower than both flat-rate ($880) and standard interchange-plus ($714 on Helcim).

The break-even for subscription vs. interchange-plus is typically around $15,000–$25,000/month, depending on the subscription fee and per-transaction rates.

How to switch pricing models

Switching processors is easier than most merchants expect:

1. Get your current effective rate. Divide last month’s total fees by total volume. This is your baseline.
2. Request formal quotes. Ask 2–3 interchange-plus processors for a quote based on your volume and card mix. Provide your current statement if possible — good processors will do a free cost analysis.
3. Check for contract terms. If you are with a processor that has a long-term contract (like Clover’s 36-month term), check the early termination fee. If the annual savings exceed the ETF, it is still worth switching.
4. Plan the transition. Most processors can set up your account in 1–3 business days. Plan to run both processors in parallel for a week to ensure the new one works correctly with your POS or e-commerce platform.
5. Verify after 30 days. After the first full month on the new processor, recalculate your effective rate and confirm it matches the quoted estimate.