Definition
A transaction fee is what a payment gateway charges per individual payment. The most common shape is a percentage of the transaction value plus a fixed per-transaction amount, but some providers (notably SumUp in many markets) charge percentage only, and bank transfer rails like SEPA or ACH typically charge a flat fee with no percentage.
Transaction fees stack with other charges — international surcharges, currency conversion margin, monthly platform fees, and dispute fees — to make up the total processing fee. The transaction fee is usually the largest single component, but on certain transaction profiles the surcharges or FX margin dominate.
Why it matters
Transaction fees scale linearly with revenue. They are one of the few cost lines that double when revenue doubles. For most merchants they are the largest variable cost after cost of goods, which makes them disproportionately important to model and review.
Where this appears in your tools
The Payment Gateway Optimizer breaks transaction fees into percentage, fixed, international, and FX components, then compares the total across providers using your real numbers.
Example
A 200 sale at 2.9 percent + 0.30 carries a transaction fee of 5.80 + 0.30 = 6.10 (roughly 3 percent effective). A 5 sale at the same rates carries 0.145 + 0.30 = 0.45 (9 percent effective). The fixed component matters more at low AOV than the headline percentage.
Common confusion
Transaction fee and processing fee get used interchangeably, but they are not always the same. The transaction fee is per payment. The processing fee is the total of every fee category the provider applies — transaction fee plus surcharges, FX, refunds, monthly minimums, and so on.