Setup Cash Payment Method (Tested in Odoo 10.0)

POS App > Payment Methods > Create New one called "Credit Card" (Type: Cash, Electronic)

Invoicing > Invoice > Register Payment > Credit Card > Choose stored token (Token storage can be done manually via Authorize.NET website and link the Payment ID under Stored Credit Cards of the Customer or via customer portal at http://<odoodomain>/my/invoices by the customer themselves [Customer Portals must be enabled in Settings > General Settings). I don't know an easy way to fill in CC info from the Odoo backend (as distinct from the portal)

 I found that if a customer signs up for the portal (using the sign up link that is unique to them -- the eCommerce app's custom email template has an example of it) and then goes to /my/invoices  then they can see all their invoices that they have there. They can then click on one, and then pay it via authorize.net

 You can only store the credit card for customers in the portal by enabling the "merchant payment method" links the "Customize" menu option in the website builder.

  'Show payment buttons to employees too' checkbox is not available in Odoo 9.0 or 10.0, which was the way people used to do it.

I'd show screenshots but there is no way to upload one apparently.