Is it forbidden not to add the extra amount —which drivers include in the car's load—?

The employee is an agent of the company, and he must adhere to honesty and not violate the explicit orders of his principal; as Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." It is not permissible for the agent to exceed what his principal has commanded him. This prohibition is even more severe if the increase is not permissible for the company, such as drivers taking it without a rightful claim.