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;

});