Many years ago, there were domestic air mail rates. If you didn’t opt for air mail, it would presumably travel by ground, and as a result, somewhat slower.
But at some point, probably 1970s or so, domestic air mail rates went away, and first class mail would travel by air if necessary.
I guess that somehow it ended up becoming more efficient, but how?
(Just asking about basic First Class mail, not parcels, periodicals, bulk mail, etc., although in some of those cases, the same reasons might apply.)
For short distances trucking works just fine, but for longer distances flying is more efficient. If you can eliminate some of your trucks by switching over to planes for all first class long distance mail that improves your overall efficiency. Getting a rate increase ensures the extra cost is covered.
Extra costs for sending ordinary letters ended on May 1, 1977:
As recently as the early 1970’s, it was still quite a bit cheaper to travel by bus than by airplane over long distances in the U.S. I know, because I took some long bus trips back in the 1970’s. Now it’s still somewhat cheaper to travel by bus but not hugely so. I presume the same is true for transporting mail across the country. I presume that it’s more efficient to have a unified mail system in which any mail traveling a long distance goes by airplane.
The main shift was from trains to planes. The Post Office Department cancelled all railroad mail contracts in September 1967, electing to move all First Class mail via air. A lot of passenger trains were supported by the mail contracts, so this decision accelerated the death spiral of American passenger trains.
About the same time, the speed and reliability of the new Interstate Highway network allowed for trucking lots of mail—even Third Class and Parcel Post—up to 1000 miles in a day.
Turning the question over, what’s today called “First Class mail” IS the old air mail.
What you can’t buy from USPS today is a discounted service that’s slower than FCM and travels by intercity truck only. USPS doesn’t sell it because it’d be vastly slower but only a few percent cheaper. In fact, given the investment they’d have to make now to bolster their truck routes, it’d probably cost more than FCM. So they figure (IMO rightly) that nobody would buy it.