I did some work a couple of years. Relatively small. 

It was about working out where the credit cards were captured and stored. And how they were communicated between different layers in the company stack. It was also about understanding the encryption and tokenising used throughout the process. Plus how the cc information was sent through to the payment gateway and beyond. 

Quite detailed and technical. 

Kimbo

Sorry it was commercial in confidence and I don't work there anymore anyway, so I don't have it.

good luck

Kimbo