how do i set up email notifications on payment due dates?

Quick Base does not provide this functionality natively.  i.e. sending a notification when a date changes.  But there are some options as work arounds to either force trigger the notification, or use on outside service to send the emails.

Set Triggers.

Using Quickbase Actions, When record is modified and after the change is made in Start Date AND after the change all of these conditions are trueStart date is not equal to Start date snapshot then check start date modified. ( check box)  

Sometimes it is important to use snapshot fields to record info you want to be able to check against. then use a check box for the trigger.

Have a checkbox with this code (modify it to fit)

If(Today()>=[Due Date],true,false)

Then simply make a notification where: 

If a record was modified, select a specific field (your checkbox) and blamo!  For QB to know who to send the message to, you'll need to link to a user on your form.  The notification will have a "Notify Whom" dropdown, and select the proper field. 

Use the same logic above for 1 week prior notification, etc.

Scott

The kicker on all of the above suggestions, is that the record would have to be modified.  Which doesn't always happen in the time window you are looking for.  So I wouldn't rely on that method.