Speaking of tax, we have a strange situation in Australia. You may notice that the stamps posted above a labeled "International", that's because G.S.T. (Goods and Services Tax) applies to domestic postage, but not to international postage. As a result, different stamps must be used for the two purposes. When purchasing stamps in Oz., therefore, you must make it clear whether you require domestic stamps, with G.S.T. included, or international stamps, on which G.S.T. is not paid.
International stamps cannot be used on domestic mail. On the other hand, domestic stamps can be used on international mail, but only if you put on 10% extra (to cover the fact that 9.09% of the money paid for the stamps will not go to the Post Office, but will have to be given to the government as tax).

