In case of prepaid tax, I think the shipping company handles clearance and taxes. They may just base that tax on the declared value of the package, instead of the actual value. Sellers often declare a low value (such as $100) to reduce the taxes.
For example, in my case (Belgium), the customs service always rejects the low declared values and stops the shipment until I provide an actual invoice with the real value of the package. Then they charge about €70-80 of fixed costs, plus 21% VAT on *everything* (incl. shipping and fixed costs). However, this process is non-transparent, adds weeks to the shipping time, and they often make mistakes