Is working in Saudi customs clearance permissible or forbidden (halal or haram), and is one who participates in it considered a helper in sin and aggression?

The basic principle of customs clearance work is permissible, and so is the wage for it, because the clearer acts as an agent for the merchant and it does not fall under the category of assisting in sin. If the customs duties are legitimate, then it is not permissible to use deception to evade them. However, if they are unjust (from forbidden taxes), then it is permissible to evade or use stratagem to reduce them.