The pricing is set up for you to pay half now and half later. If you choose to pay the full amount upfront, double the quantity per image to create the full cost of the image. Then in the notes section let me know that you are paying the full amount for ____# of images.
So if you want to order 50 images and want to pay the full amount upfront, put 100 in the quantity box. Then in the notes section of the check out process, state that you are ordering 50 images but paying the full amount upfront. :)