Decoupling In-App-Billing from Activity

Decoupling In-App-Billing from Activity

All tutorials I could find on Android In-App-Billing V3 assume you have
one single activity that handles everything billing related. In my case
there are multiple activities which will need access to the billing. How
would I handle such a thing most elegantly?
One example problem I stumbled upon: When working with the Google billing
helper classes you always hand over the current activity as a parameter.
Later callbacks (e.g. onActivityResult) are called on that activity. But
what if the active activity changes all the time? Do I have to shut down
and re-initialize the billing all the time?