You can simulate a successful Settlement flow in Sandbox by requesting a manual Settlement in Fuzebox. To do this go to Accounts> Overview> Settle account. This will always result in a Settlement in a  COMPLETED status and will generate a Settlement Notification and a  Settlement Report that will be available to be downloaded. Please note that on Sandbox, you will not be able to see the current account balance of the Volt Account. If the test fails, make sure to make a test Gateway transaction and get a RECEIVED payment status first.