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.The way you indicate FlexPay you want to run a transaction in Sandbox mode vs Live, is by authenticating your request with a dedicated API key that you generated for this purpose. See the screen below.
Please refer to the Generating Your API Keys article for detailed step-by-step instructions.
For more detailed information, refer to the Sandbox User Guide article.