Yes, any purchases count toward the collab.
I use pse 5 for some things and when I create my .jpgs to send off to print, I open up my original .psd which is 12x12 at 300ppi because I always start layouts with that. I choose Save As (not save for web) and then save the .jpg at quality of at least 10 but often go ahead and do 12 just to be absolutely sure of quality. I'll then name the jpg with something like <blah>_print.jpg to distinguish it from .jpgs used to upload to a gallery. The size will be somewhere around 3-12 megs.
The store printing products describe this process too :)
You wait until you get your prints from us! The quality is unbelievable and and we take extreme care with actual shipping so they make it to safely and uncreased!
I try to take one day at a time, but sometimes several days attack me at once.