That sucks man. I've still - knock on wood - been able to avoid that scenario but have come very close before.
How expensive is the wrong item you shipped? If it were me (on an international sale) I'd be really tempted to just eat the cost of that item, and get the correct shipment out the door asap. I'd rather shorten the lag time experienced by int. buyers due to a mistake I made than risk a case or neg.
When thr dust settles, if I felt like the buyer was OK, then I'd ask about a possible return of the first item. Otherwise, they can keep it and I chalk it up.
think i am going to try and get the item back. it can easily be sold for ~$150.
i'd rather eat the cost, as annoying as that is.