To change the plugin shipment number is possible using the filter hooks
Copy and paste this sample code to your theme functions.php file
// Filter Shipment number
/*
* Adding prefix "SITEA-" to the shipment number
*
*/
add_filter( 'wpcwp2p_generate_shipment_number', function( $shipment_number ){
return 'SITEA-'.$shipment_number;
});