Just as an update, I finally found a solution, but it wasn't what I originally thought.
I used
www.smugmug.com and completely uploaded everything up to their servers. They have a very good built-in gallery that people can also buy prints from.
I had a lengthy conversation with a representative at
www.printroom.com. They have a solution that can be integrated with Gallery as seen here:
http://phatcatphoto.com/events/galle...lubs411_sat001
PhatCat uploads the files to their Gallery on their site like normal, but there is some simple html that integrates the pricing and tells printroom the paths to the images it needs. When the visitor buys a print, it is transferred to printroom as needed. There's a technical pdf that outlines how this all works. It's not that terribly hard because I could understand it.
I would contact them to see if this same integration is possible with Photopost. I know I would've been a Photopost customer if it was available for me at the time.