How Do Older "First Class Mail" Stamps Work?

When buying stamps from a dealer recently, he only charged me 37 some odd cents per Stop Family Violence Semi-postal, 33 cents or something for the Breast Cancer semi postal, and 37 cents or something for the Toy Train First Class mail stamp. Is the implication here that that's all they cover in postage? Can't I use ANY first class mail designated stamp to always cover the modern day rate, or is it, as written on the sheets themselves, "valid for postage at the First class mail rate in effect AT THE TIME OF PURCHASE."

In essence my question is, aren't first class mail stamps (semi posts included) technically forever stamps? What's the difference?

From USPS:

So I basically have to research when they came out to see what the postal rate was at the time of issue? Seems kind of bogus

Is it unfair to abide by the conditions under which the stamps were issued?

Things don't always go our way just because we want them to.

To be fair, your user ID implies you're old enough to remember the rates, and bold enough to not be afraid of a little research.

If it doesn't say Forever it isn't a variable rate stamp.

This is a rather odd practice, which assumes either that a first-class stamp is immediately used, or that its owner can tie it to a past postage rate. Here, a first or second class stamp can be used as such, irrespective of when it was issued. On the other hand, the likelihood of a US post office employee checking the issue date and value of a stamp on a package going through the system must be low.

I use the following page all the time, which lists all of the nondenominated U.S. stamps, when they were issued, and their postal value.

https://alphabetilately.org/G2.html

As Geoff says, it is unlikely that the postal service cares anymore. You could likely use bird poop to mail a letter so long as it was on a properly sized piece of self adhesive paper.

Forgot to put a stamp on the envelope and it was received with no problem.

Stamp Smarter also has all the non-denominated stamps here https://stampsmarter.org/features/Id_US_RegB.html
Click tab named 'View non-denominated stamps'. This tool also has every US stamp listed for anyone who might be interested.
Don

But getting back to the original question: The semipostals are even more confusing than most nondenominated stamps, because they were often sold for long periods of time across several changes in postal rates. So their current postal value isn't necessarily the same as the value they had when originally sold; it's equal to the last postal value that particular stamp had when sales ended.

In particular, any semipostal that is still being sold has the same value as a Forever stamp, even if your particular copy of that semipostal was originally bought at a lower value. But once a semipostal goes off sale, its value no longer rises with future rate increases.

Clear as mud?

All of the semipostals can be used today as domestic first-class postage - it says so right on them.

Any Forever stamp is the same.

The oddballs are those that were issued just during a rate change or at a rate change that carry no denomination.

Initially everyone was so flippant towards me about their value but now I feel people are coming around to my confusion towards their supposed quasi forever status? If I was sending mail to friends I would just test out a bunch to see what happened, but as I use postage almost exclusively for mailing out eBay sales, I can't risk it.

Another question— has your postal clerk ever called your particularly heavy stamp usage "suspicious mail"? The clerk today said I had to limit myself to 2 stamps! ????

Another question— has your postal clerk ever called your particularly heavy stamp usage "suspicious mail"? The clerk today said I had to limit myself to 2 stamps! ????

Tell your clerk he/she better read the postal regulations again.....
You could put 58 1c stamps on it if you like

I paste envelopes and packages with lower value stamps. Fronts, tops and sides. Just dont put them on back. In Canada as long as the postage was never used and pays the going rate (without the surcharge of the semipostal added in) and it is properly addressed it is good to go. And as sheetguy says, tell them to read the regulations.