Do I earn commision on paid sales originating from the free hosting cPanel?

Yes however you must enter your affiliate ID in the reseller panel.  You can find your affiliate ID by registering for an IFastnet account here:

https://ifastnet.com/portal/register.php

Then goto the Affiliate page :

https://ifastnet.com/portal/affiliates.php

When signed into your IFastnet account click on the Affiliates section and you will your Affiliate ID at the end of your Unique Referral:

You can enter your Affiliate ID in the Reseller Panel under the Customizations -> My Affiliate ID section:

You can also use your affliate URL to link to paid hosting plans from your reseller home page and on Social Media posts etc.