Changing or Discount Delivery Fee on orders over X

We currently use the shortcode so that customers see the calendar and can select on our homepage what dates and times they would like a delivery. To display the costs associated with each timeslot, we have added a cost to those timeslots. But we would like to offer free delivery on purchases over $50. We can do this easily with shipping, but then customers will not see the delivery costs associated with each time slot when they select the timeslot. We tried implementing an automatic discount for a flat amount for our regular deliveries ($7.95), but then if the customer picked a timeslot that is discounted or even free already, that automatic discount still triggers giving them the free delivery + $7.95 off their order. We are looking for a way to offer free delivery over a certain amount while still being able to have the prices displayed on the calendar.