If the item has already been shipped, there is nothing anyone can do.
On the USPS Customs form, shipping is not to be included if it was paid separately. So there is no breakdown of item price and shipping on the form. Only the amount you paid for the item. If the item you bought was listed with free shipping then that is the amount you paid for the item and that is the amount that should be on the form.
Now if you are talking about some other form that I am not aware of then I can't help you there.
Non paying buyers deserve unpaid item strikes.