The easiest way is to email me stating the total number of images you wish to purchase. I will then pro-rata the difference between what you've bought and the total that would be owing.
Alternatively, when you're in the proofing gallery, top right is a cart icon where you can purchase additional sets of images, 1, 2, 4 or 9 at a time. Although a few people have stated that they've been unable to buy more than 1 at a time, this has been reported to the developers.