The term duty is from late 14c., duete, "obligatory service, that which ought to be done," also "the force of that which is morally right," from Anglo-French duete, from Old French deu "due, owed; proper, just," from Vulgar Latin *debutus, from Latin debitus, past participle of debere "to owe." Duty in international trade is from late 14c.
In economic sense, a duty is also a kind of consumption tax. A duty levied on goods being imported is referred to as an import duty. Similarly, a duty levied on exports is called an export duty. The maximum amount of customs duty in the US shall not exceed $485 and shall not be less than $25. The fee is based on the value of the merchandise being imported, not including duty, freight, and insurance charges. MPF for informal entries (i.e. goods imported via mail etc.) is a set fee and ranges from $2.00 to $9.00 per shipment. The purpose of customs duty is to protect each country's economy, residents, jobs, environment, etc., by controlling the flow of goods, especially restrictive and prohibited goods, into and out of the country.
Obligatory, counter offer and accept, require an answer