I agree with Sherry. After it's scanned at U.S. Customs, there's no way of knowing where it is after that. It looks like it's stuck in customs but it's probably in the buyer's country somewhere (or on its way). Your buyer was unreasonable to leave feedback so soon.
I'd update your policies to include approximate delivery times for international packages. I've posted in my own policies that international delivery could take anywhere from 2-8 weeks and that I have no control once the package has entered the post system. You should also have a statement about duties, taxes and customs fees being the responsiblity of the buyer.