U.S. postal rates

USPS has now made new price adjustments and restructured international shipping in May: "economy" (or boat) is not an option anymore.

The cheapest available rate for mailing books to Kosovo now is first class international. Prices vary. A softcover book weighing about 12 ounces would be $8.65 and from there up to a pound the price rises to $10.10. On the other hand, you can use a flat rate priority box and ship as many books as would fit for $37.00. A flat rate box will hold about 16 paperbacks or 10 softcovers or about 4 hardback editions. Paperbacks will get the most reading material for the money, and this flate rate priority box is a great way to defray shipping costs with a group of your friends.

thanks to Rob for the information