I would like to make my app freeware, but if a user is willing to retrieve additional features, it should be possible to ‘activate’ them after in-app payment.
Should I set up my app as “FREE” (or can that cause trouble later as this setting can’t be change anymore?) and which way would you recommend to go – should I retrieve the additional stuff from my own website, or does Google provide a way for it after payment was successful?
Basically the app will be (and stay) really useful (well I hope) in its free state already, I just want to kinda provide additional goodies for the few that would be ready to support and maybe want to get even more out of it.
Yes, set up your app as free.
What you’re describing is the freemium model, where you give the app for free, and charge for advanced features:
http://en.wikipedia.org/wiki/Freemium
A lot of Android apps are successfully monetizing using the freemium model. One example is Angry Birds Space, a top downloaded free app, which uses in-app billing to sell Mighty Eagles or remove ads.