Thanks for the input.
As I've stated above, our charges are equal too. But it means -the onsite repairs are more profitable for us. Why then start the whole bussiness with the workshop. It's usually expensive matter.

Concerning the travel time - yes this is an issue ;-), but from the other hand the customer must pay twice for the platform trailer transporting the truck to and fro.
Not mentioning the replacement truck, the customer often requires, because the workshop repairs usually take more time.