I'm building that functionality into https://giraf.app, and will set the fees as low as possible (aiming for a flat 4%, compared to Bandcamp's 15% on digital items and 10% on physical items). My goal is for Giraf to help artists by maximizing how much they take home, and by giving them a platform for building a unique kind of relationship with their fans, around their interests in music, film, books, visual art, and other forms of culture.