The plugin generated 225 characters of unexpected output during activation

I solved this issue. Problem was in way I was using register_activation_hook.

I loaded TbybInstall along with other classes:

/*
 * Load classes
 */
require plugin_dir_path( __FILE__ ) . 'classes/class.PreparedCarts.php';
require plugin_dir_path( __FILE__ ) . 'classes/class.ReturnOptions.php';
require plugin_dir_path( __FILE__ ) . 'classes/class.TbybInstall.php';

…and I used register_activation_hook in a way which is more similar to official WordPress guide:

/* Create database table on plugin install */
function create_tables(){
    TbybInstall::create_tbyb_prepared_carts_table();
}

register_activation_hook( __FILE__, 'create_tables' );

And that’s it.