What is the primary purpose of a customs union?

The primary purpose of a customs union is to allow free trade among member countries while imposing uniform duties on imports from non-member countries. This arrangement effectively eliminates tariffs and trade barriers between the members, fostering economic collaboration and market integration. By establishing common external tariffs, a customs union simplifies trade relationships with third-party nations, ensuring that all members adhere to the same regulations and duty structures when dealing with non-members. This not only promotes trade efficiency but also strengthens the economic ties between the member countries.

In contrast, the other options do not accurately capture the essence of a customs union. While enforcing taxes on imports may happen, it is not the primary function of a customs union, which is more focused on facilitating trade among its members. Currency exchange is a separate economic consideration unrelated to the customs union's core objectives. Lastly, regulating international shipping falls under different forms of agreements and regulatory frameworks, rather than the specific aim of a customs union.