This does not mean that they should turn a blind eye when facing two-sided platforms. A case in point is provided by platforms that supply a service to their members, but are not the only route for a purchase (see Figure 5). For instance, American Express provides the cardholder with a service, but other payment methods such as cash, check, or other card systems are also available. A hotel or airline flight can be booked either through an online booking platform, such as Booking.com, or directly.
Such platforms usually charge a merchant fee and demand “price coherence” (the merchant is not allowed to surcharge for a transaction performed through the platform relative to a transaction that does not use it). While price coherence has sound justifications (it prevents surcharging hold-ups by the merchant20), it also comes with hazards; for, high merchant fees are in part passed through to third parties, namely consumers who do not use the platform. This may result in excessive merchant fees21. The market failure in this instance is not the skewed pricing pattern (which is typical of two-sided markets), but the externality on non- contracting parties.
The analysis reveals that the merchant fee should obey the following Pigovian principle: In the case of card payments, the merchant fee should be equal to the benefit that the merchant derives from a card payment22. The consumer, who decides on the payment method, then exerts no externality on the merchant. This principle is now the European Commission’s doctrine for regulating open systems Visa and MasterCard.
20 See e.g. Bourguignon et al (2014) and the literature on hold-ups/shrouded attributes.
21 See Rochet-Tirole (2002) and for a recent and elegant framework Edelman-Wright (2014).
22 This principle is called the “avoided cost test” or the “tourist test” (would the merchant rather have a customer pay by card rather than cash, given that the customer is in the shop, can pay by either means of payment and a tourist and therefore will not be attracted in the future by the merchant’s accepting the card?). See Rochet-Tirole (2011).