The below constants only need changing if another plugin uses the same _GET variables to do something else – in 99.999% of cases you should be able to just leave them alone

However, if the above should be the case you can overwrite the following constants by adding them to your wp-config.php. Ensure the two values set are different from each other. !!!

a) WPPIZZA_TRANSACTION_GET_PREFIX – get parameter for successful transaction

/* wpptx used as default */
define('WPPIZZA_TRANSACTION_GET_PREFIX', 'wpptx'); 

b) WPPIZZA_TRANSACTION_CANCEL_PREFIX – get parameter for cancelled transaction

/* wppcltx used as default */
define('WPPIZZA_TRANSACTION_CANCEL_PREFIX', 'wppcltx');

constants above should be added to your wp-config.php somewhere before
/* That's all, stop editing! Happy blogging. */