I have a long-term revolving debt account where interest is charged only for the outstanding balance. I am using a credit card for regular expenses in order to maximize the duration that money stays in this revolving debt account.
One of my suppliers requires a surcharge when paying by credit card (instead of direct debit). Instead of paying C, I would have to pay 1.03C or something similar, in order to get the benefits of deferring the payment.
Assuming that my long-term debt is at D% and the surcharge is S%, and my credit card additionally offers R% in cash-equivalent rewards, what formula governs the maximum surcharge I should accept?
