As long as you have e-mail order notifications enabled in your Notification Preferences, Amazon Payments will send you a notice every time a new order is placed.
If you are no longer receiving these e-mails, you should make sure that:
E-mail order notifications are still enabled in your Notification Preferences found in the Settings drop-down menu.
You have received new orders since the e-mails stopped coming.
Any firewall or anti-spam software you have installed is not blocking these e-mails.