Online marketplaces, such as Amazon, or online travel agencies, such as Booking.com, greatly expand consumer information about market offers, but also raise sellers' marginal costs by charging high commissions. To prevent direct selling, some platforms adopted price parity clauses, which restrict sellers' ability to oer lower prices on alternative sales channels, while others rendered mandatory the use of their own marketplaces. Whether to uphold, reform, or ban price parity (or, more broadly, favor disintermediation) has been at the center of the policy debate, but so far little consensus has emerged. In this paper, we study a natural alternative; namely, how to optimally cap platforms' commissions. The optimal cap reflects the Pigouvian precept according to which the platform should not charge fees greater than the externality that its presence generates on other market participants. This idea can be operationalized by either assuming a specific demand model (logit), or by employing an approximation based on extreme-value theory. A numerical illustration in the context of online travel agencies suggests that cap regulation binds if optimally set.