You will need to integrate your Railz account with different accounting service providers before requesting your business customer to connect their accounting services.
To integrate different accounting service providers onto Railz, navigate to Integration >> Accounting on the Railz Dashboard™
As you can see in the above screenshot, you will have the option to enable the accounting service that you would like to use. Some accounting services like Oracle Netsuite and Quickbooks Desktop can be enabled simply by toggling the button whereas, some services require you to add the Client ID and Client Secret in order to be integrated.
More information on the accounting integrations can be found here.