Would it be possible to redefine "customer" to include the credit card costs? Then the MFC clause can remain, but it will have a different meaning.
Like, if the customer A pays me $10 in cash, and the customer B pays me $11 using credit card, and the credit card company then takes $1 from me, in some sense I have provided "the same price for each customer": each of them ultimately gave me $10.
Like, if the customer A pays me $10 in cash, and the customer B pays me $11 using credit card, and the credit card company then takes $1 from me, in some sense I have provided "the same price for each customer": each of them ultimately gave me $10.