How Do Older "First Class Mail" Stamps Work?

When I use a bunch of old stamps to make up a rate, the postal clerks in my town find it amusing. I think it also rankles them a bit because they have to add it up. I've never been told my usage is suspicious.

"Suspicious" because of lots of stamps??? Please, Mr. Clerk, show me in the regs where that's even a thing. I would talk to the Postmaster, or possibly some 'regional' postmaster (don't even know if that's a thing, either, but it should be) to get the postal clerk on the right page. They can't just make stuff up.

Non-Denominated stamps are valued at the rate at time of issue. Semi-postal is a little more complicated in that initially they were valued at the current rate + a minor value for donation to a specific cause. If the postal rate increased they were still valued for the current rate but less to the cause. Semi's were issued for a specific period of time & were required to be legislatured for any extensions. Forever stamps are issued at the current rate. When the rate increases they are ISSUED (sold) at the new rate but if you hold any previous to the rate increase they are now VALUED at the current rate. (did I gat this right?????)

At least in places I've worked, if we're sorting mail and we see an envelope without a stamp, we figure it's not worth inconveniencing the recipient over 58 cents.
Side-note: I've never really looked at the Semi-postal stamps before, I just figured they said forever like the "normal" forever stamps do. It could be that by the time stamps were a thing I needed to pay attention to selling books of forever stamps had become the standard.

I agree the only stamp that would cause confusion would be the US semipostals --- in particular when the USPS takes it off sale for awhile and then reoffers it as it did with the Alzheimer's stamp.

So far the USPS isn't pulling any issued from the past few years off sale.

As for using a variety of stamp values - I only have had a mild quibble with the post clerk when sending first class mail/postcard international mail. All clerks accept it but a few sometimes suggest I just use the current international first class mail stamp.