You have the option to Pre-Sell your product at an event.  This is where you will take payment before you take the photo and print the image.  This will guarantee that you will not waste any paper.  You also have the option of taking pictures of as many people as possible and then printing and assembling the product first.  Your potential customers will then preview the photo already in the product.  You will undoubtedly have many images that are thrown away but you will sell more product in the long rung because once people see themselves, their family, or their friends they will be more compelled to purchase on the spot.