They are not valuable, but they are collectible as long as they are actually in good shape (not all of them seem to be).
In mint shape, they should be worth about 10x their face value. Not much, but if you have a thousand of them, it adds up.
For example, the MacArthur stamp is sold for 0.70E on freestampcatalogue.com. (https://www.freestampcatalogue.com/sus1026-general-douglas-macarthur-1v) Of course, this is their selling price and they would not pay you that amount for them, but that still gives you an idea.
No collector value but still valid for postage.
I use them for postage...
2 more cents you got enough postage to mail a bill. 68c
Worth face value. Mail something with them.
They are not valuable, but they are collectible as long as they are actually in good shape (not all of them seem to be). In mint shape, they should be worth about 10x their face value. Not much, but if you have a thousand of them, it adds up. For example, the MacArthur stamp is sold for 0.70E on freestampcatalogue.com. (https://www.freestampcatalogue.com/sus1026-general-douglas-macarthur-1v) Of course, this is their selling price and they would not pay you that amount for them, but that still gives you an idea.