When DevMate launched, I dabbled with the backend a bit and I really liked it. They use FastSpring for the store and enabling DevMate to integrate into your FastSpring account is very simple.
It seemed to be kind of expensive, though. Since then, they changed the pricing model once or twice and made the platform more attractive to upcoming indies and teams with a lower budget. Now it's free.
Their SDK seems to provide easy integration. I think I'll check it out with my next app to tell you more about the details. I just hope they're doing well – lowering the price could just as well mean that they try to attract more devs because it doesn't pay off to run the platform at the moment. (I think I'll just ask them to be sure.)