Two parts, two behaviours
The percentage scales with transaction value — a 2.9 percent rate is 2.9 percent of every sale, large or small. The fixed fee does not scale — it is the same 0.30 on a 5 transaction or a 500 transaction.
Because they behave differently, their relative weight changes dramatically with average order value. At high AOV the percentage dominates and the fixed fee is rounding error. At low AOV the fixed fee can be larger than the percentage component.
The crossover point with worked numbers
Take a 2.9 percent + 0.30 rate. On a 100 transaction the fee is 3.20. Of that, 2.90 is percentage (90 percent of the fee) and 0.30 is fixed (10 percent). Effective rate: 3.2 percent.
On a 5 transaction the fee is 0.445. Of that, 0.145 is percentage (33 percent of the fee) and 0.30 is fixed (67 percent). Effective rate: 8.9 percent. Same provider, same rate, three times the effective cost.
Why this matters when comparing providers
A flat-percentage provider with no fixed fee (SumUp in many markets) wins for low-AOV in-person businesses. A percentage-plus-fixed provider wins for higher-AOV transactions where the fixed fee disappears into rounding.
The crossover where one becomes cheaper than the other depends on the specific rates, but for typical SMB pricing it usually sits around 20 to 40 in AOV. Below that, flat percentage tends to win; above it, percentage-plus-fixed tends to win.
How to use this when picking a provider
Calculate your real AOV from a recent month. If it is under 30, pay close attention to the fixed-fee component when comparing providers — it can swing the result by more than the headline rate. If it is over 100, the fixed fee is mostly cosmetic and the percentage is what matters.
The Payment Gateway Optimizer calculates effective rate including fixed fees automatically — you do not have to do the maths in your head. But knowing which side of the line you sit on tells you what to focus on when reading the result.