If a customer decides to pay an invoice by credit card, we apply a fee. But that fee is not on the invoice, so the payment is more than the invoice amount. The easy answer would be to add the fee as a line item on the invoice, then apply the payment. But due to terms, the invoice may be in a closed month, so can't add a line. There doesn't look to be a way to account for an adhoc charge at the point of applying a payment. Any advice?