I have had a system like this for 10 years because my customers would never get Joomla or Drupal. This is a good lesson.
While WordPress may not be the best it does get attention on Google. So we are using it more and more.
As for charging for GLP. I've been using osCommerce for years and I'm open with customers. I don't charge them for the code I charge them for all the work of setting up the site, adding modules, making modifications and importing the products. We do the same with WordPress.