Is it allowable to declare the value of goods as less than actual value for the purpose of custom duty and income taxes?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If these duties and taxes are taken to provide a given service, then it is unlawful to evade paying them. Because in this case those taxes are taken rightly.

But if they are taken as part of an oppressive scheme and a wrong system, then there is no objection to your refusing to pay them provided you do not tell explicit lies. Allah knows best.