You have already bought a premium theme?
And you need to keep your customization on your premium theme, even after updating theme?
Then child theme is the best way to keep your customization.
If you want to make a child theme for your premium theme, then you can choose my service.
My Service Includes:
-- Child Theme Making
-- Making Child theme according to client requirement
-- Will show you client how to modify (by screenshot images)
-- Child Theme will contain the customize coding.