Payment methods are implemented as plugins in nopCommerce. Payment method is an ordinary plugin which implements an IPaymentMethod interface (Nop.Services.Payments namespace). As you already guessed IPaymentMethod interface is used for creating payment method plugins. It contains some methods which are specific only for payment methods such as ProcessPayment() or GetAdditionalHandlingFee().

I has developed a free BPAY payment module which enables your customers to pay online for their orders in your nopCommerce solution. BPAY CRN number can be generate by OrderId or CustomerId, can be change from plugin admin configuration.

How to create BPAY CRN http://www.ashishblog.com/bpay-ref-number-bpay-crn-using-c/

Install Plugin

1. Clone or download the source code (https://github.com/A5hpat/Nop.Plugin.Payments.BPay)
2. Add the plugin source code to your nopCommerce solution (.sln) file
3. Re-compile the solution file, run the website, and install the plugin
4. Configure a BPAY info in the plugin configuration page (Biller Code, CRN Ref number, handling fee etc…)

5. Enable BPAY plugin.

6. Create test order with BPAY payment and see order details (Note)

Code

source code (https://github.com/A5hpat/Nop.Plugin.Payments.BPay)

Ref: http://docs.nopcommerce.com/display/nc/How+to+code+my+own+payment+method