Пошлина is a payment for state service. Tax service, customs, municipal services and so on. Тариф is simply a payment rate, regardless of where you should pay to.

Пошлина is the duty assesed by customs, тариф is the rate you pay for the service. Beware, that could be tricky and have different meaning in another countries, like UK and Australia. I was contemplaing on this subject from US/Russia standpoint

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