Whether you pay duty on an inbound package comes down to the declared value versus your destination country's de minimis threshold, which is the value below which imports enter free. That threshold ranges from generous to nearly zero depending on the country, and some places apply VAT from the first unit of value even when duty itself is waived.
The calculation is usually based on the declared value of the goods, and in many countries shipping and insurance costs are added to that base before duty and tax are applied. On top of that, couriers commonly charge their own customs brokerage or handling fee for doing the clearance paperwork, which surprises people more than the tax itself.
To estimate your exposure, look up the customs authority's threshold for postal imports and the standard VAT rate. Keep the sender's invoice or a proof of value, since providing documentation quickly is what gets a held package released. Underdeclaring is not worth it; penalties and delays cost more than the duty.