As I've been integrating cfpayment with my commerce application, I've found it handy to have a diagram showing the important bits of the system. This diagram (click the thumbnail for a legible version) shows the bean-like methods in brown, and the methods that would be used by an application using the software in red. The methods in plain black are interesting, but should not have the focus. I should note that the diagram is not a faithful UML Class Diagram of the software, obviously. Some private properties and methods are left out for (an attempt at) clarity. There is also no such abstract class as cfpayment.model.Account; the credit card, EFT, and token only inherit from it by convention. At any rate, I hope it's helpful.
While working on the PayPal Website Payments Pro gateway for the cfpayment project, I had to fiddle with the typical unit testing procedure. The PayPal Express Checkout process involves a user interacting with the PayPal website between the start and completion of a payment transaction. The documentation at PayPal has a pretty good overview of the flow, from the two supported entry points.
If you look at the source from Subversion, you'll see that the NVPGatewayTest.testBeginExpressCheckout() method first sends the SetExpressCheckout command to PayPal, then redirects the user to the PayPal site. Once the user authenticates at PayPal and confirms some profile information, they are returned to the unit test at NVPGatewayTest.testCompleteExpressCheckout(), bring along two URL parameters: token and payerId. The test then performs two requests of PayPal: GetExpressCheckoutDetails and DoExpressCheckoutPayment.
This seems to be working pretty well. I recorded a screencast of the process showing the test from start to finish: PayPal Gateway Express Checkout Testing.