Hi Scott,
There is actually separate "Add for Additional Item" for Domestic vs. International for Flat Rate in Auctiva Shipping Profile. In fact, that "Add" can be done for each selected shipping service/method.
Also, don't confused "Combined" rule with "Add" rule. Add is done for a purchase of multiple items on the same listing. Combined (and promotion) rule is used across multiple listings/purchases at checkout. Only combined (and promotion) rules are setup in eBay account preferences and synced back to Auctiva. When you want a combined (or promotion) discount rule to apply to a listing, you check that box on the one-page lister.
Hope that helps.
Danno