Answers
What does it cost to use my own couriers?
Nothing per delivery. Your platform subscription covers dispatching to your own couriers. A per-delivery fee applies only when you draw on the freelance network.
In more detail
This is the single fact that changes the arithmetic. Marketplace commission scales with how much you sell; a subscription does not. The more you deliver with your own drivers, the lower your cost per delivery goes.
The per-delivery fee exists for the deliveries your own drivers cannot cover: the unexpected rush, the driver who called in sick. You pay for capacity at the moment you need it rather than staffing for a peak that happens twice a week.