@Ashley we would always apply the discount to the order subtotal. We only offer those terms to customers who pay for the entire invoice amount in one payment, and we would only offer it to customers who pay by EFT or cheque where we aren't also being charged transaction fees. We find payment terms quite difficult to keep track of given that there's no indication on the ship schedule when a customer is expected to pay their order balance prior to the order shipping out. What we have started doing is setting up all of our 'pay first' customers with a tiny credit limit (our go-to is $0.25 but anything small would work). This at least gives a warning on the ship schedule if a customer has not paid off the balance of their order. It would be nice if there was a better way to handle the pay first customers.
I agree with @Geoff Kessler... for our customers who send us a cheque for their statement amount at the end of the month, there's no good way to set up their invoice due dates to avoid having interest charged when it shouldn't be. We would appreciate the option of net 10th as well.