Are you considering developing a payment terminal or enhancing an existing plugin to function as a payment terminal.
Our need is to push an a customer balance and a few hidden fields as invoice #, Customer name/address to a customer payment terminal whereby the customer may enter credit card info to make the payment for the completed balance or may enter a different amount to pay. The vendor would receive email notification of the payment with all the data fields of the payment terminal & last 4 digits of cc and the payment logged in the plugin. A range of payment gateways would be nice, but as minimum authorize.net and paypal.
You have a couple plugins that are close but not are can do these basics