Could you clarify how the billing works for writable storage and bandwidth.
Will I just get charged automatically for what I use if I use more than the free allocation, or can I/do I have to specify how much I will use?
We aren’t charging for bandwidth anymore, so that isn’t a concern. Regarding writable storage, once you’ve used over the free amount, we’ll start charging at the rate of $1.50/GB per month. You don’t have to specify any sort of max, it dynamically grows with your app.
Is that possible to tell the app to not use more than the max free amount? I don't want to be charged a lot after a digg effect or whatsoever...
Currently no, but that functionality will be available in the new dashboard.
what will be the behavior of the app then ? Overwrite oldest files ?
You will be able to specify what amount of disk space you want available. If that disk space runs out, your app will no longer be able to write to it. It will be up to you and/or your application to manage file storage within your shared writable directories.