Promotion/Coupon limited to certain shipping methods/rates

[djsparks]

I am trying to add either a promotion or coupon for free USPS Media Mail shipping for orders of $50 and above. I have tried setting up both promotion and coupon and attempted to limit them as desired. Unfortunately, the discount applies to both Media Mail and Priority Mail (both live rate shipping methods under USPS). How can I make this discount apply to only one shipping method?