I use my site for x amount of free downloads for certain packages. In order to download for free you have to use promotions. The way this works is make your image available for download for say 10.00 a piece. Then if you are giving away 10 free images that would be 100.00. You then give the client a promotion for 100 dollars off of the subtotal which zeros out the balance. You can still checkout at a 0.00 balance if you use the email checkout option however this will not work with paypal, or google wallet as they do not allow a 0.00 balance since they cant charge you a percentage of 0.
Hope this helps you!