Allow discounts simply by paying with a specific payment gateway.

Under Settings > Company > Payment Gateways, add a new sub-link called "Discounts".

In this section, list all enabled gateways, and allow a % discount to be set for each gateway.

This is useful for gateways like BitPay, vendors will often allow discounts for paying with gateways with little to no risk.

The discount should be applied at payment, and perhaps a % discount shown for the payment gateway during checkout.

( ) PayPal Payments Standard
( ) BitPay (Automatic 20% discount)

Since the discount is applied during check out perhaps we should record 2 transactions. The payment transaction, and then an "In house credit" or new payment type of "credit" for this purpose. This way the invoice is paid in full, and does not need to be adjusted. So, when choosing to pay with a specific gateway that has a discount, we show the discount, charge them the amount with discount, and apply 2 transactions to the invoice. Alternatively, we could add another negative line item for the difference. - Maybe this is better.

Needs more information.