There are a lot of cPanel resources that you can search and download from PLRdatabase.
Check these out – and you have Private Label Rights for them! It may be something your customer may want to purchase from you.
https://www.plrdatabase.net/product/cpanel-power-package/
https://www.plrdatabase.net/product/hosting-panel-secrets/
Also, if they use WordPress (highly recommended!) then you may want to check out the following (also with Private Label Rights):
https://www.plrdatabase.net/product/advance-wordpress-mastery-kit/
https://www.plrdatabase.net/product/advance-wordpress-mastery-kit-upgrade-package/
Good luck and best wishes!! :)