Yes, but you have to determine when the stamp was issued and therefore the denomination. If the stamp was determined to be a $0.39 cent stamp, you would have to put on on additional $0.03 cents for the current $0.42 rate. Only the liberty bell "forever stamps" are non-denominational and therefore good even if the USPS increases their rates.
Yes. But that's more postage than a standard letter is worth. You can add small-denomination stamps to bring the total up to whatever the first class rate is when you use the old stamps. First class stamps in 2006 (Lady Liberty with a US flag backdrop, or two blue lovebirds on a yellow background) were worth 39 cents.
The USPS is still issuing new first-class stamps and has no plans to stop - stamps sold to collectors and not used for postage generate a significant amount of revenue. Perhaps you are referring to the fact that the government no longer prints the stamps ; they are printed by private printing companies.
The US Post Office continues to make 4 cent stamps today. 4 cent stamps were issued to cover the first class rate of postage from 8/1/58 to 1/6/63.
The 5¢ Franklin and 10¢ Washington stamps
An album of old stamps
you can go to a shop that sells stamps
The USPS often requests a rate hike. They do not know if they will be granted the full amount, or if it will be something less. In order to prepare for the new rate, they print up hundreds of thousands of stamps, but they don't know what value to print on them! So they put on a letter instead. The H Rate Makeup Stamp is to be added to the value of the old First Class stamps and bring them up to the new First Class rate. In this case they are worth 1 cent of postage.
There are a few websites where an individual can purchase old stamps from Australia. Examples would include Atoncer, Ebay, and Collectible Stamps Collection.
That depends on many factors. A stamp is worth its face value for use as postage. If you have something that is old and is rare, it could be worth some money. Today's First Class postage stamps are 44 cents.
It depends on the country in which you live. In the US stamps never expire, so the answer is yes. ( There are a few old exceptions, but you are not likely to have any of these lying around. Also special delivery stamps which did not really pay for postage, but for a service that is no longer available, are not valid for anything anymore.)
One of the best places to buy old British stamps is online on Ebay. Also the Arthur Ryan & Co. located in Richmond, Surrey is a good shop where you can buy old british stamps.
Yes, they can be added together. The USPS often issues a 'make-up rate' stamp that can be added to the old first class rate to make the new rate. And the US has not devalued the postage stamp since the US Civil War.