We consider a platform that helps consumers more easily discover and transact with suppliers. Such platforms have come to dominate many sectors of the economy, raising issues about the high fees they charge suppliers, especially since they tend to commoditize the suppliers they aggregate. We show that in a baseline setting, the welfare-maximizing fee exceeds the platform's marginal cost by the extent to which suppliers obtain lower markups on the platform than in the direct channel. We examine the robustness of this simple principle, and explore factors that make the efficient fee higher or lower than this level.