1) custom: 

A duty or tax imposed on imported and, less commonly, exported goods.
关税：对进口货物且很少对出口货物征收的一种税

2) duty
Abbr. dy.

A tax charged by a government, especially on imports.
缩写 dy.税：一种由政府对进口货物征收的税，尤指进口关税

3) tariff

Duties or a duty imposed by a government on imported or exported goods.
关税：一国政府向进口或出口的商品所征收的一项或若干税种

custom 有 "习惯" "海关" (customs) 之义

custom duty: 关税

在经济合同中 可用 "tax and duty" 表示 "税金"

P.S.货代考试教材中的“进口关税”是import duties

reference: http://cache.baidu.com/c?m=9f65cb4a8c8507ed4fece7631046893b4c438014639c91572c85c45f93130a1d5a31febd2520170ed1c0776d16af3e06acaf22376a4376b8cd8fce01cabbe36e7cc87c6024&p=882a9646c78a12a05ebcce121564&user=baidu