In most cases the tax is collected at the moment some shipment arrives at ports
. Governments normally force tariffs (or import duties) to protect local industries and to raise their revenues, although many economists have debated against it. According to them these methods are faulty, because in the end it's the consumer who suffers at the hand of high prices and inflation. As an exporter it would be better off going for some country with minimum tariffs because you will loose the low cost advantage once you have to pay these taxes. Tariff allows local manufacturers to offer lower prices as compared to the imported items.