fredriksthlm
So C3 will be able to use any keystore file. When you do a build via the build service it will ask you for a keystore file along with it's alias/key password/store password. We're avoiding storing the keystore itself within the editor. The hope being it will encourage users to put the file somewhere safe/secure and not just leave it in C3, trusting it will always be there. So no "import" is required.
If you want to the editor will also be able to generate new keystores for you. One limitation is that you will not be able to sign an existing APK, it will only work with builds.
Bootfit
The fee unlocks the ability to publish applications to the play store and allows you to use services like In App Billing and Play Games.
Google does not issue you with a certificate, and you don't need to give it to them. You create the certificate on your computer, use it to sign the APK and then give the signed APK to Google. The first time you upload an APK for a new application they read the signature in the APK and that signature is permanently associated with that application. To publish new versions of that application the signature must match the original one, so you need to use the same certificate every time. Once a APK has been signed it cannot be modified, or it will appear as tampered with and it will not be run.
To generate a keystore you need to use keytool which is part of the Java Development Kit ( JDK ). If you don't like using the command line you can use Android Studio to create a keystore for you.
There is a little bit of information on https://developer.android.com/studio/publish/app-signing.html#signing-manually about creating a keystore using keytool and signing an APK using the apksigner tool.