It would be a kindness if you'd just send a short note to let me know who you are, and what your interests are. Thanks.
Clearly, the purpose of many station owners in building their stations was to promote their other businesses. Hence, announcements to the benefit of the owners would essentially be considered a commercial announcement, even though no money actually changed hands. However, it didn't take long for paid commercial messages from other businesses to appear.
The orthodox answer to the matter of the first paid commercial has been that it was developed by WEAF in New York by AT&T in August of 1922. The book "The WEAF Experiment" by an AT&T employee describes the concept of "toll broadcasting" as it related to sponsorship of whole programs. (The first sponsor, and hence commercial - according to AT&T - came from the Queensboro Corporation of New York, to sell real estate. The set of five programs over five days starting 8/28/22 cost $50, plus the long distance access fee.)
AT&T even went to court to try to enforce their right to control all advertising. The court battle raged and AT&T lost. Imagine if ever advertisement generated a line on your phone bill!
However...
In March 1922, in Seattle WA, Remick's Music Store (which published and sold sheet music) sponsored a one night a week program on station KFC, co-owned by an electric shop and the Seattle Post-Intelligencer newspaper. Remick's supported the show with big ads in the newspaper-- inviting people to come in after the show and purchase the songs the Remick's Singers had just performed. This seems to predate the WEAF broadcast by almost six months.
On April 4, 1922, car dealer Alvin T. Fuller purchased time on WGI, Medford Hillside, MA and did so again several more times. But by the middle of April, the District 1 Radio Commissioner, Charles Kolster, had written WGI a cease and desist order, since at that time, doing "direct advertising" was not permitted, according Herbert Hoover's interpretation of the DOC Regulations.
There were other stations also reporting they were selling time for announcements.
And - in 1893, the Hungarian telephone "broadcast" service reportedly sold 12 second spots between the news and musical segments for the equivalent of (US) $0.50.
The first "trade-out" spots were likely broadcast on Herrold's KQW in the mid 19-teens. A music store owned by Wiley B. Allen provided the recordings Herrold used; in return KQW told listeners where they could purchase them.
By the way, David Sarnoff liked to claim credit for proposing the use of radio broadcasting for business purposes. Sarnoff wrote a 15-page document foreseeing a business of selling "radio music boxes." Sarnoff, however, did not envision transmitting advertising messages by radio broadcast. In fact, he wanted radio broadcasting to be an educational and entertaining medium only, paid for by the makers of receivers out of profits they made selling his "radio music boxes."
There is also a report that the "Jersey Review," a newspaper, purchased time on January 1, 1922 on station 2IA.
In 1966, Gordon McLendon bought KGLA(FM), Los Angeles and changed the calls to KADS(FM), running only commercials: local ads, commercials and national commercials.. It was targeted to grab business from the newspapers. The want ads were either their own voices into a telephone recording device or they could use professional announcers.
In August 1967, McLendon declared it to be a failure; returns to "regular programming". The decision was made to change call signs to KOST-FM and play "beautiful music."