title: WooCommerce Add Fee If Coupon Applied
body: <p>I'd like to be able to add a fixed $20 fee if a given coupon code (100% off) is applied to the cart. The fee should also not have taxes applied.  I'm offering a try at home service for $20, so a certain coupon would discount the cart and all products by 100% but add a $20 fixed fee.  Any help is much appreciated!</p>
  
  <p>JC</p>