How to order prints of different photos from Aperture in one order?

Hey there. So I like a photo and want to order a print of it to be sent to me through Aperture. So I go to File / Order Prints and select the size of the photo I want. But before I complete my order I want to add a print of a different photo too. I can't seem to be able to do this. Anyone know if I can?

I don't want to make a different order for each photo because then i have to pay shipping cost each time.

Thanks in advance!