It would be difficult to determine without seeing the item. In general, being on a partial is not going to add any value to it. However, there are specific cancellations and postmarks that might have a value. Your best bet is to educate yourself about cancellations and make a determination on your own.
Certainly, that is what they were created for, to serve as postage. Just make sure the total face value of the stamps equals the appropriate rate for the envelope or package.
Yes. Postage does not have to paid through individual stamps; as long as the value of all stamps placed on the envelope is enough to cover the cost that envelope carries, any number and value of stamps can be used.
It means that your return envelope must have stamps stuck ON IT to the value of 1.05.
US postage stamps do not expire -- they are always worth face value as postage.
No. It's enough to write "Par Avion - By Airmail"clearly in the top left hand corner of your envelope.
It depends a great deal on the stamp and how old it is. If there is a special cancellation or cachet on the envelope, it is better to keep it on the envelope. The older the stamp the more likely that the envelop will have more value than the plain stamp. It is better to leave the stamps on the envelope because there is a postage mark that goes over the stamp. That will give the stamp more value.
"A " stamps are the same as 15-cent stamps.
Flag stamps were 42 cents face value, they were supposed to be forever stamps, postage never to increase from that 42 cents, but that flew out the window real fast.
Face value for postage remains valid until the government de-values them. In the US, stamps issued since 1865 are still valid for postage. The value to a collector varies based on supply and demand.
$12.50
It may have value if you can find someone to buy it from you. A postage stamp has value, but not anything that you can demand. You can't hand one to a clerk in a store and they have to take it.
There are many factors involved to determine cost. Stamps of this era were produced in large quantities. If the stamps are on envelope, they could be worth more than a mint or unused stamp. Consult a catalog to identify the stamp and to determine a value. You can find them at your library.