Paypal system which is integrated in Sitebuilder performs payments directly through Paypal itself. So transaction will be successful if Paypal give success result.
You can check success payments by the status of the order in E-shop. When payment is done status will be changed to 'Paid'.
Status should be changed within 3 days, which required for Paypal to make a transaction. As I know 3 days this is maximum period.