They get a significant markup for providing this "service" to the receiver, so it is not in their interest to help the sender. More charitably the actual duties to be paid might not be known until the package reaches the border at destination.
> They get a significant markup for providing this "service" to the receiver, so it is not in their interest to help the sender.
I understand. It is a service, and I am willing to pay for it. The alternative is that I don't send presents with them. "Happy birthday! Quick pay 20 bucks before you can get your present!" is not really a good experience.
> More charitably the actual duties to be paid might not be known until the package reaches the border at destination.
I understand that too. That is why they are sending the request for the duties only once the package is at the border. But why can they send the request towards the recipient and not towards the sender?