In order to get free shipping you will need to meet the free shipping thresholds for all of the warehouse groups that your cart is made up of. Here is a list of all of the shipping thresholds for the different shipping groups:
ADSCheap Warehouse Group A: $49
Warehouse Group B: $59
CBI Warehouse Group C: $59
Promo Products Warehouse Group F: $99
Example:
If you have items from Warehouse Group A and C in your cart, you will see them separated in the cart and you will see how much more you need to add to each warehouse group in order to qualify for free shipping in both groups.