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.

The websites that list the non-denominational stamps that were posted in this thread are very valuable as a resource. Like a G rate 1st class stamp is worth 32 cents.

One way that I explain the whole situation of the 1st class rate stamps is this- the usps doesn't know exactly how much the postal rate commission will allow the postage rates to go up by, so prior to forever stamps, they would make transitional stamps that will sell at the new rate after it is determined, as the PRC could take months figuring it out, until they can print stamps with the new rate value. A first class letter when the usps was formed in 1971 was 8 cents. When the rate went up to 10 and then 13 cents it caused some problems with stamp design timing and printing, which is why like the 1975 non denominational christmas stamps exist. The next 8 rate increases after that (15, 18, 20, 22, 25, 29, 32, and 33) were represented by the A-H rate stamps. After that, instead of doing I, J, K, L, M etc they'd issue a stamp that just said First Class or something along that line until they printed the same stamp design but with the proper postage rate like 34 or 37 cents. The forever stamps eliminated the need for this. I think they stopped doing the letter rates (A, B, C etc) because it confused clerks and customers alike as well as international markets.

I read that the USPS stopped at H because they didn't want to issue an I-rate stamp.

Robert