I'm not a fan of money in general, but there comes a time in every software engineer's life when they have to deal with purchases, and that's when the game gets *real*, because it directly connects to your customers' money so your reputation is on the line. Not to mention your own wealth and sanity (if you leave a security hole you can easily become bankrupt). I'll tell you an interesting story about Android In-App purchases. Along the way you'll learn important security principals of in-app purchases and e-commerce in general.
This won't be any kind of marketing presentation: you will hear fair amount of criticism and frustration.
We'll have accompanying source code again.