You will receive a notification whenever a payment process is completed, successful or not. GlobalPay will send a POST message to the Notification URL you initially setup in the Merchant Dashboard.

To ensure that a payment has been securely processed you will need to validate the authorization header using your SiteID and API Key in the same manner you compute the authorization header for a request. For more details, please go to Authentication section.

Also, you will have to whitelist Smart2Pay’s IPs for Test and Live Environment.
Here you will find the complete list of GlobalPay’s Environments and IPs.