What if I get the coffee for free because I helped her with an assignment? What if I get the coffee for free because she thinks then I'll vote for her in a party preselection for delegates to the state convention? (Both of these have happened with me).
Commercial vs noncommercial is actually very hard to distinguish some times, and business at the higher levels is less about cash and more about relationships.
(I've never paid cash for sex, but I've written papers, offered sympathy, helped with chores/moving, taken out socially, given tours of my city, in more ambiguous circumstances)