Every card payment carries a real processing cost. Stripe's published direct rate is 2.9% plus 30 cents. A platform transaction fee at or near that level is essentially passing through the cost of moving money.
The number to compare against is always the processor's own published rate. A platform charging 3.9% on its own Stripe account is adding roughly a full point on top of what the processor charges, which is a payments markup rather than a pass-through.
A transaction fee that falls as you scale behaves like a cost line. One that stays flat or rises behaves like a revenue share wearing a different name.
The difference between 3.9% and 2% on $3,000,000 of annual billings is about $57,000 a year, for an identical payment moving through an identical card network.
Transaction fee, in practice
Benchmark it against Stripe's published 2.9% plus 30 cents. PharmaBro charges 3% on Launch, 2% on Grow and 1.5% on Scale, so from Grow onward the fee is below the processor's own direct rate.