Koumuin de Antai no Hazu ga, Jikobukken Tantou ni Idoushimasu

Aire, a working-class girl with a unique aesthetic sense, was making a living by delivering mail. Searching for a certain item, Aire attends an auction held at a nobleman's mansion, where she discovers that a forgery has been mixed in with the merchandise! This incident draws the attention of the young Marquis Bartrud and his family, who run the auction—!?

(Source: Kadokawa, translated)