Likely part of a letter to Secretary Heath endorsing the magazine, they read, "I am indeed more than pleased with The Numismatist and think it is the best publication of its kind." In October 1903, that journal reported a change of address for Mehl to Box 24, Alvord, Texas. In 1903, he joined the British Numismatic Society. Mehl's first mail-order advertisement appeared in the December 1903 issue of The Numismatist, selling 33 U.S. coins on a highest-bidder basis. Q. David Bowers, in a 1999 article on Mehl, found it logical that Mehl would focus on the mail order business given Fort Worth's distance from numismatic centers such as New York and Philadelphia—there would be few local collectors to patronize a coin shop in "Cow Town", as Fort Worth was known. By this time Mehl had returned to using the address on Main Street in Fort Worth. The sale was likely not a success, as most of the coins were listed on a circular Mehl sent by mail in January 1904, but the following month, he placed his first full-page advertisement in The Numismatist. According to numismatic author John N. Lupia III, "Apparently he was a smashing success from 1904 on." Mehl was aided in becoming a major coin dealer by C.W. Cowell, a Coloradan who was selling his collection, who sent many rare pieces to Mehl on consignment, allowing him to offer rarities that otherwise would have been beyond his means. The February 1904 advertisement offered The Hub Coin Book for sale for $.25 (), although in an unfortunate misspelling in the ad, the "k" in Book was rendered as a "b". This book, published by Alexander & Co. of Boston, was widely-sold, and coin dealers could order copies imprinted with their name and address for sale to the public for a small fee. Mehl did so, and sent out copies advertising himself, seeking to promote his business. In 1906, he published the first edition of The Star Coin Book: An Encyclopedia of Rare American and Foreign Coins, sold for ten cents and heavily copied from The Hub Coin Book. Heath described The Star Coin Book as a "well illustrated sixty-four page pamphlet giving prices he pays for coins, particularly the American series, and much other valuable information". Beginning in 1908, Mehl published the more thorough The Star Rare Coin Encyclopedia, which by 1924 had an annual sale of 70,000