For most items airmail postage and firm packing is US$2.80 to anywhere in the world.   More than one small item (such as stamps) will go together in the same envelope for the one lot of postage.
Where the items to be posted are larger (such as covers or large stamp blocks or artwork), then the cost of postage is shown in the item description.   But US$2.80 is the minimum cost to anywhere in the world except for Australia where the cost is US$1.50.