Is it Islamically permissible to save a sum of money at the car customs with the help of an employee who knows me, so that the additions present in the car are not registered?

It has been previously clarified in answer no. 33853 when the collection of customs duties is a legitimate right that should not be evaded through deception. If those conditions are met, then what the questioner did is impermissible. However, if the situation is otherwise, which is often the case, then collecting customs duties is an unlawful appropriation of people's wealth, and there is no harm in using stratagems to avoid paying those funds, or some of them.