MB3 101b: Asking a Non-Jew to do Work on Shabbos (307:4-5)

Hiring a non-Jew to buy something for you is permissible if you don't' specifically ask him to buy it on Shabbos. What if market day in your town is Shabbos?

Amira L'nachri is permissible if the activity is a shvut for a sick person, a pressing need, or for the benefit of the public (שבות דשבות)

When is שבות דשבות permissible?