Billing is an integral part of any undertaking and ERPNext Healthcare achieves this by making use of the ERPNext Accounts module.
Note: All transactions of a Patient is booked against the Customer which it is linked to.
All ERPNext Healthcare documents which require Invoicing will have buttons which would take you to the Sales Invoice with the Items configured for the service. You can then proceed by following the ERPNExt Accounts module workflows. Please note that your User account should have appropriate privileges to access the Accounts documents.