I'm intrigued by this type of case - this article doesn't mention any specific legislative requirement that fees should have been less during the period concerned. It's a strange precedent that a company can be retrospectively liable for charging "too much".
This is similar to the case of Apple and their tax dues in Ireland.
Maybe someone with a better grasp of legal matters can explain a mechanism that gives this case a chance. Otherwise I can't see this claim succeeding without causing a great deal of business uncertainty.
It's not that unusual in the antitrust context. The whole theory of antitrust is that anti-competitive behavior allows a company to charge prices greater than they would be able to charge in a competitive market. If a company is found to have engaged in anticompetitive conduct, it's common in a civil suit for the measure of damages to be the difference between the prices actually charged and some judicially-determined market price (usually based on an econometric expert analysis).
This makes sense - I think the Reuters article was quite misleading. I posted another comment [1] with a link to a Guardian article that supports what you wrote.
My guess is they are trying to get it labeled as an invisible tax because a mastercard transaction fee wouldn't have shown on the receipt or the customer wouldn't have known they are paying higher prices due to the transaction fee.
This is similar to the case of Apple and their tax dues in Ireland.
Maybe someone with a better grasp of legal matters can explain a mechanism that gives this case a chance. Otherwise I can't see this claim succeeding without causing a great deal of business uncertainty.