We've checked your customer site and the attached doc, we're ready to create a plugin as per your requirement.
Please come over a discussion so that we can chat about functionality of a product designer is for every product and a live preview.
We've created 15+ WordPress plugins on the freelancer platform, here are some examples:
https://www.freelancer.com/projects/PHP-Wordpress/wordpress-plugin-wanted.html
https://www.freelancer.com/projects/PHP-MySQL/Wordpress-Plugin-Required.5453620.html
https://www.freelancer.com/projects/PHP-Wordpress/upgrade-plugin.5741059.html
Hope you'd get an idea about our capabilities.
Looking forward to hearing from you soon.
Regards,
Arpit.