Actual Postcard Rate Within US In 1978

While I was searching for examples of the US 1757c stamp (13 cents, issued 1978-06-10) usage on postcards, I've noticed multiple rates (9, 10, 13, and 15 cents) on postcards mailed within US in 1978.

The USPS site provides the history of the postal rate for postcards, for 1978 it was 9 or 10 cents (raised on May 29).
https://about.usps.com/who-we-are/p...nce-1873.htm

So the question is -- what was the actual rate for postcards in 1978?

A potential explanation of the differences is that the sender didn't care whether it was 10 or say 13 cents if a 13-cent stamp was at hand (and USPS didn't care either since the rate was paid anyway).

Another explanation might be additional services that were upgrades above the base rate, but I have no info about this.

If you are going to dig deeply into US rates, then Beecher & Wawrukiewicz's "U.S. Domestic Postal Rates" is what you want.
Yes the domestic U.S. postal card rate went from 9 to 10 cents of May 29, 1978 (and the letter rate went from 13 to 15 cents) Finding a 13 or 15 cent stamp on a postcard is usually just a convenience overpayment, and would not get any different service or upgrade of any kind. Oversized postcards would need the letter rate, so that is where to look for your CAPEX stamps on cards ... or out-of-period use during Nov 1, 1981 to Feb 16, 1985 when the card rate was 13 cents.

Thank you, John.

Since the CAPEX stamps were issued on 1978-06-10, and the rates were updated to 15 and 10 cents for letters and postcards respectively on 1978-05-29, I'm not sure what the in-period proper use would be. I guess the 13 cents rate was for the special services/upgrades, and for that info I need to obtain a copy of Beecher & Wawrukiewicz's book.

Also, I'll keep looking for the convenience overpayment specimens.

Sorry, but I believe you are overthinking this.
The CAPEX issue was primarily a philatelic issue and did indeed come out just after the rate change of the letter rate from 13 to 15, so the issue was (in a sense) obsolete. It would have paid the rate for additional ounces on letters beyond the first ounce (15 cents for 1st ounce, 13 cents for additional ounces). Here is a flat postmarked in November 1978 weighing between 7 and 8 ounces sent between two collectors using a full block of 8:



0.15 for 1st ounce
0.13 for each of 7 additional ounces
=1.06 total, with 1.08 paid as a convenience of using an old 4 cent stamp from the 1960s.

Does the CAPEX issue exist on a card? Sure, but it overpays the rate by a few cents. Good luck in your search.