My company is a service based business, so when I send an estimate to a customer, I have a custom date field for a reservation date. For example, The customer is requesting my services for an event at some date in the future. If the customer accepts the estimate, it converts to an invoice but the due date is set to On Receipt. Only the Retainer invoice is due on acceptance, the balance invoice should be set to a date relative to the event, like -5 days.