There are two 'taxes' which must be considered: import duty and sales tax (e.g. IVA, GST, VAT).
In MOST countries, the assessable value of imported goods for the purpose of determining import duty is the FOB value.
In MOST countries, the assessable value of imported goods for the purpose of determining sales tax (IVA in the case of Chile) is CIF (i.e. FOB plus insurance plus freight), plus any import duty paid.
Thus, the poster's comment
makes sense in this context.However, somebody at the aduana decided to add (!) the charge for "flete" ie the cost of the Fedex shipment, to the $30 US value of the contents. Suddenly there is IVA due, plus various other little items, totaling about 10.000 CLP.
Aduana was just carrying out the common practice of determining value for the purpose of assessing for sales tax.
Of course, actions vary between countries and individual officials, especially in Chile. Sometimes the amount of customs duty and sales tax is too small to bother with because it costs more to administer than what is collected, and they ignore them; sometimes not.
I hope this clarifies it.