Transaction Fees: The Hidden Cost That Makes Platform Economics Counterintuitive

The Intuition Is Usually Wrong

When engineering teams first evaluate the cost of a SaaS e-commerce platform versus building custom, the initial instinct is that the SaaS platform will be cheaper. The logic seems sound: avoid hiring developers to build what Shopify has already built, pay a predictable monthly fee, move on.

This intuition is wrong for businesses above a certain revenue threshold, and it's wrong in a specific way that the platform vendors don't advertise prominently.

The Anatomy of Platform Costs

Any honest platform cost analysis has three components:

Base platform fees: the subscription cost for the platform itself. Shopify Plus runs to a few thousand pounds a month, depending on the plan.

Plugin and app costs: the ecosystem of third-party tools needed to close the gap between what the platform provides and what the business needs. For a subscription business: a subscription management app (such as Recharge), A/B testing, cancellation flow automation, referral programme management. Each is a separate monthly subscription, and together they add up quickly before transaction fees.

Transaction fees: here is where the economics break decisively. Shopify charges an additional percentage on every transaction when you use a third-party payment gateway. The rate depends on your plan and has changed over time, so check current pricing; the examples below use an illustrative 0.5%. On £1M of monthly revenue, that's £5,000 a month; on £5M, it's £25,000 a month. These fees are in addition to your payment processor's fees.

Side by Side

Laid side by side, the two approaches look like this:

Every line except one stays broadly flat as the business grows. The custom build starts with a higher engineering cost; the platform route starts with plugin costs plus a fee on every order. At low revenue the platform is cheaper. As revenue grows, the transaction fee line keeps rising until it outweighs the extra engineering the custom build needs.

The Compounding Effect

The cost of transaction fees grows in step with revenue, while engineering cost stays roughly flat.

Engineering cost doesn't scale with revenue; it stays roughly constant. So the higher the revenue, the wider the gap between the two approaches.

This creates a counterintuitive outcome: the "cheaper" SaaS option becomes progressively more expensive as the business grows. Percentage-based fees become a growing cost as your revenue scales.

Why This Isn't Obvious

The transaction fee impact is easy to underestimate because it's not visible as a line item in the way that software subscriptions are. It shows up in payment processing reports, not in the SaaS invoice. Teams doing platform evaluations often look at the SaaS subscription fees and compare those to engineering costs, and the comparison looks reasonable. They're not counting the transaction fee.

The other reason it's underestimated: at low revenue levels, 0.5% is genuinely small. At £100K/month revenue, transaction fees are £500/month: easily absorbed. The fee is designed to look reasonable at the scale where you start, and reveal its true cost as you grow.

The Decision Threshold

The crossover point (where the higher engineering cost of a custom build is offset by the transaction fee saving) depends on your revenue and your specific plugin stack requirements. Below the crossover, the extra engineering cost of a custom build outweighs the fees saved. A leaner plugin stack or a larger engineering team moves the crossover higher; a heavier plugin stack moves it lower.

Below the crossover: Shopify's economics work. The lower engineering overhead outweighs the transaction fees.

Above the crossover: the transaction fees dominate, and the economics shift decisively toward a custom or hybrid approach.

The calculation is simple. What's your monthly revenue? Multiply by 0.005. That's your monthly transaction fee to Shopify. Is that number less than the incremental engineering cost to build what Shopify would provide? If yes, Shopify wins. If no, build.

The Lesson for Platform Evaluations

Any platform evaluation that doesn't include a transaction fee projection (calibrated to your expected revenue at 12, 24, and 36 months) is incomplete. The fees that seem negligible at current scale may be the largest line item in your platform cost at the scale you're planning for.

Build this projection explicitly, in a spreadsheet, with optimistic and pessimistic revenue scenarios. The number will tell you the right decision more reliably than any capability comparison.