Why was my international order charged customs fees and/or additional fees?

USPS makes the recipient responsible for customs duties and taxes.

According to the USPS website: “Post Office facilities must collect a Postal Service fee from the addressee for each item on which customs duty or Internal Revenue tax is collected. The Postal Service fee is authorized by international postal agreements to reimburse the Postal Service for costs it incurs clearing items through customs and collecting customs duties at the time of delivery.”

Some countries also charge a flat handling fee, e.g., the Royal Mail sets an approx. £8 handling fee for every imported package.