Yes, Mint does charge its customers, but their customers aren't who you think they are.

They automatically suggest [= scales with no marginal effort] financial products which they claim are good fits for the users (who are not the customers). For example, they might say "You're paying $X in interest a month with your current credit card Y. If you were to switch to credit card Z, you would only be paying $Q, saving you $P a month."

Credit card Z then pays them an affiliate commission for the lead. The issuer of credit card Z is the actual customer.