1) Third-Party Goodwill Cunting: The Goodwill Cunter is doing the shagging as a favor for a third party (not the one being shagged). This is often done to facilitate sex between other people (e.g., having sex with a hot girl's ugly friend so your buddy can bang the hot girl) or to distract the one being shagged (e.g., shagging a girl so her little sister can sneak out with the rest of your friends).
2) Direct-Beneficiary Goodwill Cunting: The favor is for the one being shagged. This type of Goodwill Cunting is often motivated by pity, performed by girls on really sweet friend-zone guys who never EVER get laid.
2) Man, I feel so bad for Bobby! He's so sweet and never gets laid. Since he did my homework all year, I might just throw down some goodwill cunting if I can't get my ugly friend Jeni to fuck him for me.