Free shipping by specific methods

[gomod]

I need to set up free shipping for customers who choose Priority Mail but not for customers who choose other methods of mailing. The module only seems to allow one to set up shipping by how much money they spend, not by what shipping method they choose. I don't see a way to get around this using coupons, either, since coupons only seem to allow for all shipping to be free. Any suggestions? :'(