FlexPay offers the Sandbox feature which simulates transactions in a testing environment without generating a "real" transaction with the Merchant bank. Typically, before an Account goes "Live", transactions are tested to ensure everything is setup correctly. In Sandbox mode, specific credit cards and amounts are used to generate pre-defined results which are indicators that all is good.
To run test transactions in the Sandbox environment, log into your Account
Switch the toggle button at the top right, next to your username, from "Live" to "Sandbox".
When you are ready to run actual transactions, simply switch the toggle button back to "Live". A "Live” transaction will generate a real transaction with the merchant bank.
For more detailed information, refer to the Sandbox User Guide article.