Answer
All US postage stamps issued since the Civil War (i.e., 1861-65) are still valid for postage at the price indicated on the stamp. So yes, you can still use a US stamp released in 2002.
Special Delivery stamps are no longer valid. They never paid postage but paid for an added service that is no longer available.
Here in the UK collectors often use old (1971 onwards- decimal currency) stamps for postage- I recently received a package with 20 different values from 5p to 22 1/2p on it ! All perfectly legal. Stamps are officially legal tender in the UK- you can pay a bus or train fare with them, theoretically- though I wouldn't like to try it with most of our bus drivers....
Finland
You use the postage of the country of origin, in this case, you would use Canadian postage.
A metric gram would be precise enough of a measurement for a stamp.
The US has issued many dozens of red 3 cent stamps. It will take a bit of work to identify which stamp you are referring to. I would recommend spending some time looking at a US Postage Stamp Catalog to identify the specific stamp. The bonus is that most catalogs will give you an idea of value.
According to the USPS website, under "Buy Stamps", then "Rate Change", the brown background non-denominated US Flag stamp is $.41. The US flag is one of the most popular subjects for a Postage Stamp and can be found on dozens of them. The value of each will be dependent upon condition and rarity. You would have to identify the specific stamp to determine the value.
It is in various rates, stamps will cost either a few cents to a few dollars. about 42 cents a post stamp
First class postage was 29 cents that year. A post card would be 19 cents.
The D stamp would have a face value of 22 cents. There was a make up rate stamp with a value of 2 cents.
76
Millimetres
A postage stamp in Germany during 1937 would have cost approximately £3.47
It would have been 32 cents on that date for first class postage. A post card would be 20 cents.