I think that the easiest way for you to implement this would be to alter the existing shipping rules that you have in place. For the trigger you should add a new condition, you will use AND at the beginning and for the condition you would use subtotal of the entire cart is < 75. Adding this extra condition to the trigger should ensure that the shipping is not charged if the total is $75 or greater. You would need to manually change this back after the special is over.