Interesting post! I don't fulfill via Zenfolio, but would like to at some point, so I'll probably have the same issues. The selffulfillment set-up seems a little tedious at least for common print products. Too much info and set-up. How about:
1) A Generic Price List using the Mpix Price List as a template for selffulfillment. You would also need the abiltity to create new price lists using existing custom price lists.
Workarounds:
1) Since you like to follow-up with your client You could keep your price list simple and then follow-up with your client via phone or email to get additional details and items for each order (i.e. metallic, large prints, etc.) Maybe just have a 4x6 and 5x7 item
2) There could be a vendor level function at Mpix to send all orders, or at least local orders, to the photographer