Governments and their agencies have rather odd ways about them - they always want money up front. If I am receiving an article in the USA, if the item has sufficient value I'll receive a notice from the USPS to pay duty before being able to take possession of the item.
Also apropos of the subject - if I'm on a flight that is about to land in Europe there is NO mention for passengers to fill out any customs (duty) forms.........but, before landing in the USA, US Citizens are given customs forms to fill out.
I also used to notice that Lufthansa would charge Germans more for a ticket than they charged people domiciled in the USA (frequent flier miles were also calculated differently).