How can a customer modify their invoice amount to include a 3% processing fee?

How can a customer modify their invoice amount to include a 3% processing fee?

I understand that Zoho is not prioritizing automatically a fixed credit card processing fee, but is there functionality that I can use now to allow my customer to calculate this fee for themselves and "overpay" my invoice to account for the credit card fee?

When I have tested this functionality, it appears I can change the payment amount to be less than the invoiced amount, but not more than it. It will always automatically revert back to the invoiced total when I try. Help!