You can deliver your goods to other cities and receive building materials as a reward.

Planes that make deliveries to other cities are located at the airport next to the city center. Tap on the plane to open a window that will show you what goods are needed to fill the orders. Place them in the cargo hold and the plane will take off. When it returns, you will find building materials in each compartment.

You can upgrade your planes. Each upgrade will decrease the travel time or increase experience points. Please note that it's possible to get only three planes.