Payment notifications are messages that are handled on two parts: messages received by the form owner and the ones send to the form user (buyer). Go to the Settings → Payments section of your form, and under Notifications set the way you want to receive the email notification, as follows:
The Notify me when user submits form option will trigger the email notification right after the form user has submitted the form. This does not mean the payment has been completed, so in order to receive the notification only after payment has been completed there is the second option
Notify me only after payment has been processed. Even if the form user has submitted the form, you will not receive the email notification only after the form users completed the payment process.
You can opt to trigger a customized message to your form users after they have completed the payment process on your order form. Tick the option Email users after their payment has been processed and click on the Customize message link to create a notification message.
Message customization. Type in the text box the Notification subject and create your message in the Notification body text area. Buyers will receive an email containing this message after purchase.
In the end, if you use a WebHook on your order form , you can trigger it only after the payment is completed. Just tick the option to enable it.