Fake company used as a cover by CONTROL agents.
Maxwell Smart poses as a salesman, with the Chief as its owner under his cover name of Harold Clark [numerous original series episodes].
The first mention of the greeting card company comes in 1967 in a conversation between Max and his landlady Mrs. Macklin where he also introduces Agent 99 as "Miss Primrose, who takes care of Easter greetings, and Dr. Bascomb as "Mr. Pepperfeathers", who's in charge of births and bereavements [Episode #62: "Witness for the Persecution"].
Prior to this it is evidently Max's practice to give a different cover story to different people, resulting in some confusion when he can't remember what he has told his visiting Aunt Bertha and Uncle Abner [Episode #12: "My Nephew the Spy"].
The name Potomac Greeting Card Company appears in 1970 when "Harold Clark" is arranging a safe deposit box at the Bank of the Potomac [Episode #137: "Do I Hear a Vaults?"]
When CONTROL is revived in the 1990s, the greeting card cover is likewise revived, with Max as the owner and his son Zach claiming to be "Executive Vice-president, Western Hemisphere" [GS95 Episode #6: "Wurst Enemies"].