Patrick,
Thanks for your explanation. I just tried to look quickly at our code to see if there would be a simple way to do this. If you only want 'Same Page' to work on your shopping pages, and they all have a similar URL, like /cart or even index.php?option=com_xyz, we can likely help you with a minor code change to determine if the user is on one of those pages and leave them there if so. If not, the user would go to the pre-defined login page.
For something more custom where many pages have different redirections, that would be more difficult.
I've added this idea to our investigation/to-do list for a future release for more flexibility in this type of scenario, but not sure when/if that will be available.
Thanks,
Alex