Is the financial amount given in exchange for assistance in reducing customs duties considered a bribe or unlawful (haram)?

This issue is known as "Thaman al-Jah" (the price of influence/status), and scholars have differed concerning it. The more correct view is that if the person with influence needed expenses, effort, or travel, then it is permissible for him to take a customary wage for it; otherwise, it is not. It is apparent from the question that the questioner did not exert effort or spend money, so it is not permissible for him to take the amount. This is all assuming that customs duties are unlawful. However, if they are lawful, then it is not permissible to evade them, nor to assist in that, whether for a commission or for free.