Steven Zahm@ Patrick
The error message might not be showing the complete path but if it is, the path that is being generated by WP is ./class.customizer.php (according to the error message. Technically this is correct, but I would expect the full absolute path rather the the relative path.
Add the following line above the require_once line. Save and upload the file and then click the Customize button.
var_dump( plugin_dir_path( FILE ) . 'class.customizer.php' );
What is the message it outputs? It should be the full absolute path to the customizer file.
If it is the absolute path then there is something with the permissions of the file which is not permitting it to be read. There is nothing I can really do about that. If that is the case and you are not able to resolve the permissions issue the just add // at the beginning of the reuire_once line or delete it. It is not necessary for the template to function as it simply adds a few more customize options.
Hope this helps!