There is a way in Rune Factory to be able to duplicate items that you receive during Bulletin Board quests. So you need to be given the item before the quest finishes, so it works well for delivery orders.
Having accepted the quest, play up to the point at which you are given the item that you want to duplicate.
The item is currently in your inventory. Go back to the Bulletin Board and forfeit the quest. You will still have the item in your inventory, and can take the quest again to get another copy of the item.
You can effectively keep this up until you finally choose to complete the request.