If you want to use PayPal Sandbox with Web Form Builder, there is a small change that you will need to make to one of the PHP scripts. Please note, this does not work if your form is hosted on S-Drive at this time.
Edit the file fbapp/php/checkoutcontroller.cls.php
Now comment out line Line 14 and uncomment Line 15.
Line 14 //define( 'PAYPALWPS_URL', 'https://www.paypal.com/cgi-bin/webscr' );
Line 15 define( 'PAYPALWPS_URL', 'https://www.sandbox.paypal.com/cgi-bin/webscr' );