Need to create a proofing workflow

Hello,

I’ve been tasked at an internship with finding a simple, easy means of allowing customers to upload artwork for their product order, have one of our vendors create a proof and then have the user approve/deny it.

I have searched high and low for proofing software that meets our needs but I believe being a middleman puts us in a small segment here.

Here’s a flowchart my supervisor made:
http://i.imgur.com/JNRFB.jpg

If you have any recommendations on custom content types, plugins I could tweak, or services I haven’t found online I would really appreciate it.

thank you!