Although this may have been the first advertisement purchased via radiotelephone at the Atlanta Constitution, there was at least one predecesor, as the Detroit News reported receiving a radiophoned classified ad request in September, 1920.
Atlanta Constitution, June 15, 1921, page A6:

First  Ad  Over  Radio-Phone  Received  by  Constitution

    The Constitution received Tuesday evening the first paid advertisement ever sent by radio-phone, when J. C. McQuiston, advertising manager of the Westinghouse Electric and Manufacturing company phoned in his copy from the radio station of the Georgia Railway and Power company.
    This is a novelty in the field of ad transmission, and is one of the real developments of the seventeenth convention of the Associated Ad Clubs of the World.
    When the delegates meet in general session Wednesday morning at the Wesley Memorial church, they will receive a greeting through a radio-phone, which is erected on the church rostrum from Rev. Hubert Cowley-Carroll, who will send his message from the Georgia Railway and Power company.
    Mr. McQuiston states that the transmission of news and ads to newspapers throughout the world will be common in the future. Already the news "broad-casting" station established at the Westinghouse Electric and Manufacturing company in Pittsburg sends news to various journals in Florida and the Western states he said. "The advantages of radio establishments throughout the country is obvious," says Mr. McQuiston. "It increases newspaper efficiency, gives direct communication to farmers regarding weather conditions, makes easy the speedy apprehension of criminals and supplies entertainment to the bed-ridden.
    "When Mayor Key, of Atlanta, spoke on 'The City Plan' a few months ago in Pittsburg before the Civic club, his speech was transmitted by the Westinghouse station to distant points of the United States; the same was the case with President Harding's inaugural address."
    Mr. McQuiston gave another instance of the good that can be accomplished through radio. There was a sick woman in Vermont, who had been kept in bed for 26 years, and just recently, by the erection of a radio station at her home, she was able to hear a sermon, the first she had heard, mind you, in 26 years. In Pittsburg we send out every Sunday evening the sermon of Dr. Van Etten, pastor of the Calvary church, to people who have radio stations, and especially to churches where a pastor can not be obtained."
    Mr. McQuiston added that "in the future news of prize fights, baseball games and music of the grand operas will be received by newspapers and even in private homes, instantaneously with the event."