Dynamic Invoicing - Allow customers to choose payment method and then adjust invoice accordingly.

We would like customers to be able to chose their payment method after receiving their invoices. This way we can add a convenience fee of 2-3% if they choose to use a Credit card rather than a debit card or ACH. We can currently only add a convenience fee if we know what payment method a customer is going to use in advance; however, if we send a customer an invoice and they change their mind and use a Credit card instead of ACH, we would have to send them a second invoice, which would infuriate many customers.