Are the aforementioned actions—such as not declaring the correct quantities of goods, and paying money or offering gifts to inspectors to reduce customs fees and increase profit—considered unlawful in the work of a customs clearance agent?

If customs duties are in exchange for services rendered by the state, then it is not permissible to evade them or assist in their evasion. It is permissible to work as a customs broker to pay them for a fee. However, if customs duties have no مقابل (equivalent service or benefit), then they are among the forbidden مكوس (unjust taxes/levies). In this case, it is permissible to pay money to reduce them or to evade them as a دفع للظلم (repelling of injustice). It is permissible to appoint a customs broker for this purpose. There is no harm if the broker achieves this through lying or paying a bribe to the employees, and the sin is upon them.