Provide invoice details via (Billing) API using a ServiceID w/ Billing/Viewer

Can you pls. extend the (Billing) API to allow retrieval of the index of invoices as well as the attachments (PDF, PDF-extended, XLS) via the Billing API using (!) a ServiceID w/ API key and "Billing/Viewer" access policy assigned?

Right now a individual user (IBMid) as invited member user of the IBM Cloud account is required to retrieve the data, which is not the proper way to get things automated. The "ibmcloud" CLI doesn't offer an easy way to download the invoice details (but a nasty "ibmcloud sl call-api SoftLayer_Account ..." -- which, again, requires a user ID and doesn't work with a ServiceID, unfortuantly).

In our (use)case, we would like to retrieve the new (recurring, new) invoice details of a large (>> 50) number of IBM Cloud accounts (IMS account IDs) in an automated way. Logon to all those accounts once a month and to download the details via UI is not an option!

In a second version it would be good to be able to retrieve the extended details* in a more convience format (JSON) as well.

[*] extended details of an invoice, as provided by "ibmcloud sl call-api SoftLayer_Billing_Invoice getItems --init <invoice-id>"