I'm curious, do you know of any existing invoicing software that handles discounts that way? I'm sure it exists, but I've never seen discounts that are tied to the date paid...
http://www.freshbooks.com/blog/the-best-invoice-payment-term...
https://www.americanexpress.com/us/small-business/openforum/...
Being able to define your payment terms is important.
I'll check out quickbooks.