I had a similar situation, ordered a set of earphones (domestic store) and they showed up as expected. A week later, another set shows up; check my card, no extra charges so I figured it was a mistake and sent them an email.
They didn't even know about the error, but requested I send them back and they'd reimburse me the freight charges. So I did, and sent them my account details, never saw the money for postage though.
In hindsight, I'm not entirely sure what the moral of this story is, but just let them know of their error and see what happens.