Resources Transportation mission allows transporting resources among your domains.
The mission is launched from the Kingdom Map screen after clicking the city-destination, which has to be not your current domain.
The slower speed of the mission is, the lower its cost will be.
In order to launch the mission you need to have:
Mission Withdrawal. "Resources Transportation" mission can’t be withdrawn.
Merchants fatality. Merchants can die after mission completion with 3% probability. The load is guaranteed to be delivered anyway.
Interface features. Point at the building and in the pop-up window you will see how many Merchants are trained and how many of them are in city.