I first want to say that I think your script is a great add-on for anyone doing e-commerce and should be an already built in feature. I have not yet fully tested it but so far it is looking great.
I just wanted to pass along a feature that would be nice to have incorporated in to SHP, the allowing of using customer names OR customer groups. I also have the add-on "Separate Price Per Customer" installed which allows the creation of user groups so it would be nice to choose groups instead of each individual user.
I would even be willing to try and incorporate it myself but I am not a PHP expert so I am not exactly sure which code and in which file I would need to edit.