The .25 cents Lou Gehrig Stamp was issued by the US postal service on June 10, 1989. I'm not sure what you mean by unopened pack but, collectors typically buy collectible stamps in sheets, or "plate blocks" of 4. The plate block includes the plate numbers from the printing process.
A quick search on eBay shows full sheets of 20 only selling for $5.00 (face value) to $15. Plate blocks are selling for $3.-$5.
The post office also sold framed cachets with the stamp first day issues that will sell as a display piece for about $20. -$30. First day issue cachets about $5.-$10. each
The value of an unopened pack of Lou Gehrig postage stamps can vary greatly depending on factors such as rarity, condition, and demand among collectors. It is recommended to consult with a reputable stamp collector or appraiser to get an accurate valuation of your specific pack.
US postage stamps do not expire -- they are always worth face value as postage.
"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.
Ordinary stamps were 3 cents each in 1916 .
The F series stamps were issued in 1991. The postage stamp has a face value of 29 cents.
15 cents.
You can use forever stamps for an ounce of postage forever. They never lose their value for one ounce of postage.
Forever stamps have the value of the current First Class Postage Rate. Currently they cost 49 cents. They can be used 'forever' to mail one ounce of US Postage.
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.