I appreciate you clarifying what you're looking for in this sort of situation and it's awesome that you're offering your customers discounts like these.
What I recommend in this instance is using fields such as the Customer message box on your invoice to share these kinds of details with your customer. You can also consider adding custom fields through form template customization to show your customers what their total would be if they paid by a certain date versus another. In both instances, it would mean filling out the details manually each time, however, it would allow you to ensure your customers have the information they need so they can make payments accordingly.
QuickBooks Desktop's form templates allow for flexible customization options and I encourage you to review the following article to see what you can create for your customers: Use and customize form templates
At this time, QuickBooks Desktop doesn't have a built-in feature where you can add a discount to an invoice that disappears after a certain time frame. What'll have to happen in this instance is you either have the discount included on the form from the beginning and then edit the invoice if that time passes and the customer doesn't pay, or you can have the discount shown at full price with the additional fields noting the discounted rate, then add the discount if the customer does pay within the allotted period.
Let me know if that make sense and if you have more questions.
