This question covers two topics:
1 – Customs and duties i.e., taxes that are charged on goods and products when moving them to one country to another. Taking these taxes from Muslims is expressly forbidden. This is the maks of which the Prophet (peace and blessings of Allaah be upon him) said: “The one who collects the maks will be in the Fire.” Narrated by Ahmad, 16553; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 3405. see also question no. 25758.
It is prescribed to avoid paying this forbidden tax by whatever means possible so that it will not harm him or cause more damage, even if he does that by means of a trick or paying some money to avoid this injustice.
If someone were to say that this comes under the heading of a person giving up his rights in return for money, that is permissible. This is what the fuqaha’ call waiving one’s rights. The Maalikis said that it is permissible to receive payment in return for the right to pre-emption and the Hanafis allowed waiving rights to appointment to public office and forfeiting the right to custody in return for money. See al-Mawsoo’ah al-Fiqhiyyah al-Kuwaitiyyah, 4/243, 32/83.
The answer is that this is not the same as that, because those who are exempted from paying duties are the members of the diplomatic corps themselves; they are not allowed to exploit this favour to make money by allowing other people to avoid paying duties. Using it to allow the goods of others to pass into the country is a kind of trickery and deceit. If this is permitted, it is only permitted in cases of necessity, not for one to make a profit from it.
There follow some points of advice:
1- Those who work in the diplomatic corps should help their brothers for free, because that comes under the heading of warding off injustice from them. They must do this as much as they can.
2- If they do not want to offer this service for free, then it is not permissible for them to take more than costs of shipping the goods, accompanying them, doing the necessary paperwork, etc.
We also remind them that they have to be kind to their Muslim brethren and have a good intention in doing so; their aim should not simply be to amass wealth. And Allaah is the Source of strength.