Set Free Shipping levels (dollar range) by CUSTOMER TYPE
For instance, retail customers free shipping at $50, wholesalers at $300. The most basic shopping carts have this feature.
Vladimir Mityukov commented
An option to enable free shipping for combinations of "Minimal subtotal" + "User membership".
For instance, for consumers, Free Shipping at $75 total order. For wholesalers, $300 for free shipping etc.