All perfect praise be to Allah, the Lord of the Worlds; may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
The above question indicates that the contract concluded between the above company and the transport companies is a contract of Ijarah* for work and the subject of this contract is transporting a particular substance.
From the perspective of Islamic Sharia, it is permissible for the servant (Transport companies) to contract with other servants (Truckers) via a parallel Ijarah contract to transport the substances agreed on with the hirer (Company) to make a profit out of the difference between the price in the first contract and the second. Imam Al-Khasani (May Allah have mercy on him) said: "It is permissible for a servant to work himself or use other servants, if it wasn`t conditioned in the contract that he does the work himself. This is because work is the subject of the contract and a person could do it himself or use others to do it." {Bada`ei Al-Sana`ei, V.4: 208}.
In conclusion, it is permissible for the transport companies to contract with the truckers to transport the above substance to make a profit out of the difference in the price between the two contracts, which is one JD. However, this is provided that the company hasn`t stipulated that the transport companies should do the work themselves. And Allah the Almighty knows best.
Ijarah* "To give something on rent" or "providing services and goods temporarily for a wage."