Just to go Seller Admin > Cart Shipping Settings, click Next to proceed to the Shipping Rules screen, and add a new shipping rule applying a Flat rate of 0.00 when Min. Item Total = 50.00. You may also want to delete your current shipping rule for 0.60 per ounce and then recreate it with Max. Item Total = 49.99, so buyers over $50 won't be confused by seeing a choice of two shipping rates available in their cart.