Almost all countries have issued square stamps. At some point or another you can find a square. The US has issued a large number of square stamps.
361 Countries,
Stamps can be square, rectangular, pennant-shaped or various other shapes - and have been at some time or another, considering all countries in the world. But they are overwhelmingly square or rectangular, yes.
The country of origin.
you can buy coupons at the post office that are redeemable in foreign countries for stamps
Greece has issued such stamps. There may be others.
Depends where you are. Some countries issue stamps for first class mail, some don't.
That is practically impossible to answer. There are nearly 200 countries in the world, with a wide variety of postage stamps for all kinds of categories of postage. Postage has changed through that time, with new kinds of postage and all sorts of changes in stamps. Some countries did not exist in 1960 that did in 2000, and others ceased to exist during that period. To list all the prices of all of the stamps in all of those countries and their changes, would be vast. Outside of postage stamps, then there are all kinds of other stamps, like rubber stamps and metal stamps, stamps on products etc. They all have costs too that have changed. So your question has a vast list of answers, that would be impossible to give you.
"Postage stamp countries" are tiny countries "the size of postage stamps" but that is not how they got that name. These countries issue postage stamps as a source of revenue. Most of their stamps are bought by worldwide stamp collectors rather than being used for delivering domestic mail. Four European postage stamp countries are: • Liechtenstein • Luxembourg • Monaco • San Marino
If you mean stamps for items of post you are sending to other countries, yes, Australia Post has a range of Australian International postage stamps, which are marked accordingly, in blue on the edge. If you mean stamps from other countries, no, Australia Post is not a commercial outlet for the post offices of other countries. Some of the larger Post Offices may stock packets of used (or CTO) postage stamps from other countries at minimal cost. The packets are usually focussed on a specific theme such as dogs, trains, birds, etc.
Yes, it is one of the Postage Stamp countries that obtains a large percentage of their revenue through the sale of postage stamps.
yes
Some examples of squares are chessboard, carom board, square rubber stamps, and tiles on the floor.
It depends on the issuing authority. The US has not devalued postage stamps since 1862, which means anything issued since then can still be used for postage. Other countries have invalidated stamps as of certain dates or events. Check with you local postal authority for specifics in other countries.