Sometimes, in PayPal order forms, when the option Deliver only after payment is checked in Settings → Payments → Notifications, the submission does not arrive in your inbox at all. In order to fix this, please disable the option Auto return to website after payment in your PayPal settings.
To do this, log in to your PayPal account, click on Website Payments then go Website Preferences. Click on the Update button.
Under Website Payment Preferences set the auto-return on Off.