Here's the dealio:
You never authorized that company to pay your customs duties. Since they performed you a service before you paid and before you told them to do it, they can do nothing but try to chase after you for the money.
If they took close to a year to even send you the letter, chances are that they're not so interested in chasing after you, and they're just trying to sucker those who think that they're obligated to pay into paying.
Just wait it out and see what happens. Since the company's in America, it will be even harder for them to get money out of you.
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