Yes, there are a number of issues that have stamps with horses on them. Many Christmas stamps include toy horses, there are cowboy and cattle related stamps with horses being ridden, including the Pony Express and similar historical vignettes.
Some are Scott Numbers:
2756-7-8-9 - Sport Horses
2391 Toy Horse
1528 Horse Racing
The US Postal Service has issued many stamps depicting National Parks throughout the United States. So far there are not stamps issued for state parks. It is not likely that there will be, unless there are other reasons for honoring the site, such as it being a battlefield or historical location.
No. US stamps are good for the U.S. postal service only.
No, that is the job of the U.S. Postal Service. The USPS is now a semi-independent agency. The Postmaster General is no longer a cabinet position and the President has very little say about the postal service.
The ones purchased from the US Postal Service include the stamps.
One can buy any kind and any number of stamps using the US Postal Service website. One has to go the official website and search for the right stamps.
Not in the United States. The US Postal Service approves the postage stamps.
Semi-postal stamps are stamps that are issued to pay postage, but cost more than the standard postage rate. The additional money is contributed to charity. Many countries have issued such stamps on a regular basis. The US has only issued a couple such stamps.
The price of a US postal service postcard stamp is 27 cents.
US postal stamps can only be used for letters that are mailed in the US. If you are mailing a letter in a European country, let us say France, they you need to use French postage to mail a letter to the US or any other destination.
No. The USPS is a private corporation.
No, you would need to get a stamp for the country the letter is mailed from.
No, you may not. You can only use Grenada stamps when in Grenada. The rules are set by the Universal Postal Union.