What is the simplest way to charge money over the Internet?

I have a .Net 2.0/3.5 WebApplication.

I want to be able to take money over the internet for my service.

Each of my customers will have an AccountNo.

I wish to offer several products each of which will have fixed price.

I need for my customer to login to my system and elect to pay me money for a product of their choice and for my app to become aware when this is complete so that I may adjust their account to reflect this.

What is the simplest (least complicated coding) way to take their money?

Paypal, NoChex, Google checkout, Something else ?