I don't know if this is only new for books or new in the US or if I somehow hit a trial of this feature, but I've had this buying a music CD from Amazon UK. I hit the usual default "Buy" button in the buy box and the CD was shipped from a third-party seller in another country.

Kind of annoying as it was a Father's Day gift (so the order must have been about a year ago) and the unexpected international shipping meant it didn't arrive until after the day itself. I'm pretty sure there'd been no indication of this on the sale page.