A similar idea was hinted in the past:
This is something that could be forbidden by the provider. E.g. Paypal has a policy that a vendor must not pass the transaction cost to buyers.
However what some do to circumvent that is to declare those cost as arbitrary payment gateway usage ( ) and make the buyer pay the full charge anyway. Not nice but ok…
But as I understand the OP it is not just about payment processing charges, it is the overall costs that small customers are accounting for and that they don’t bear their share.