Dunno of any apps that properly integrate into the system.
Most apps don't even bother trying to do things properly.
(i.e Integrate into the settings - Microsoft ones are ironically the only ones that do).
I really don't like DRM (But because the apps are so cheap I might not be too bothered.)
I certainly don't want my card attached to my phone or tablet at all though (Regardless of anything else).
(I would probably buy a gift card probably ever app I would want would cost less than £10).
I would buy from Amazon if they forced dev's to keeps the Apps up-to date. (Not interested in Googles awful customer service to users).
When it comes to games I have tons from all the Android Humble Bundles. (Actually half decent games compared the junk that allot of people seem to like anyway).
Square Enix these days are totally awful. (As a kid I imported a US Super Nintendo to play Final Fantasy III (Jap VI) / Chrono Trigger - the games cost £70 each - don't remember the console cost). I don't mind paying for them again but I want them so I can play it on a plane. (The modern stuff like Chaos Rings looks awful).
I really should avoid Square Enix completely but I like Chrono Trigger. (And still FFVI)