For a previous campaign where I used BackerKit, I operated an opt-out system for International Delivery, so that backers had to actively take responsibility for their delivery. I liked this principle.
Unfortunately that method was rather clunky and had some issues in implementing.
Regrettably, I am going to have to give International Backers responsibility for their delivery from their outset and use an Opt-In system. This is because it is the better option, on balance, as it is far easier to manage and leads to less confusion with Backers when paying for delivery. Backers choosing Tracked delivery then pass the responsibility for delivery back to me.