The conch shell can be found on the south end of Hustle Beach. The top of the mountain refers to the mountain at the Golden Ruins. You will find a shell style bowl at the top of the mountain that you need to place the conch shell on. You can complete 50 deliveries with a gold rank to unlock the UFO, which is the best vehicle in the game.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUISTHMzTXDO...