To change the default trip code that is generated by the plugin you can use the  filter given below:

wp_travel_trip_code

To execute this you need to activate the child theme. You can create the child theme manually or simply create the child theme using the plugin given below:

WP Child Theme Generator

After activating the child theme in your child theme functions.php add below given code.

function wp_travel_child_change_trip_code( $trip_code, $post_id ){
return 'YOUR TRIP CODE - '. $post_id;
}
add_filter('wp_travel_trip_code','wp_travel_child_change_trip_code', 10, 2);

Here replace ‘YOUR TRIP CODE” with your tripcode. Each trip will have the unique id behind but will have the same code that you enter here.