The phrase "when the single coachman becomes the substitute" appears to relate to a story or manga with a specific narrative context. In this story, the single coachman, who normally drives magic carriages, takes on the role of a substitute driver. This leads to interesting twists and developments in the storyline, including a suspicious request to head to a wedding venue within a fantasy or omegaverse setting. The story involves themes of drama, romance, fantasy, and has elements of comedy and smut as well. In essence, when the single coachman becomes the substitute, it sets the stage for unexpected events and character interactions that drive the plot forward in the narrative context of this manga or story. This role change triggers new dynamics that are central to the unfolding story.

If you are looking for the story itself or more detailed plot points, it is available as a manga titled "What Happens When the Single Coachman Becomes the Substitute." The full chapters are accessible online for reading.