It can be more complicated than the first guys seem to think. At least when shipping to the U.S. from abroad, your package, if the declared value is high enough, will be examined by customs and the buyer will be notified of the customs tax. If he refuses to pay YOU, THE SELLER, WILL BE CHARGED
1) original customs fee +
2) if you decide to have article returned to you, you pay shipping + CUSTOMS IN COUNTRY OF ORIGIN for the re-entry of the product OR
3) you decide not to...