I think a tax is any compulsory fee levied by the state. Even if you generalise the concept to all compulsory fees, I don't think it fits in here. You're free to not use the card brands payment networks, and pay them no fees. You can say that it's not practical to do so because that would severely limit your ability to pay for goods and services, but that's what transactions were like before the card brands innovated the industry. They render a service that provides extraordinary convenience for consumers, that's what you're paying for. It's no more a tax than any other expense that's factored into the margins of the goods and services you buy.