Do UPS have a service for this? Is there anything on their website? Couriers seem to be sending invoices for duty/ handling/ admin out when imports are brought in either as purchases or gifts. They should have details to calculate costs based on value and what the items are.
just sent something to the States - after considerable time sourcing Customs Invoice forms etc, it turned out the UPS labels covered it all anyway (booked thought ParcelHero). You get them to print after booking the collection...