I am using Ubercart 2.9 with Drupal 6.
Is there a way to add an extra fee to a certain product that has one value for domestic and a different value for international?
You can try UC Multiprice
This module allows you to set a different product pricing for each enabled country in Ubercart. It also got support for roles, currencies and i18n
Its only in dev version use at your own risk.
On the fee admin page, admin/store/fees, add fees with the required additional amounts. Add two conditions to each fee: that the product is in the cart, and that shipping address is [your country] or [your country] and condition negated.