Ok, if you already have the bill.com app, you will need to contact them to have them cancel your $39 account and detach your app.  Then, you will be able to use the link to signup for the $9.99 integration for bill pay.  I would verify with them the features you may be using in your current bill.com account prior to canceling just to make sure you won't lose any functionality you are using.

To contact bill.com, just sign into your bill.com account and click help or support and look for the link to chat with them.