Mock adwords API

I want to test my code that connects to the AdWords API, without making an actual call to Google (that costs money ;) ). Any idea how I can plug in a new implementation of TrafficEstimatorServiceInterface?

AdWords Client API is using Guice for dependency injection, but I'm not sure how I can get the hold of the Injector in order to modify it?!

If it helps, this is how I get the implementation of it now:

AdWordsServices adWordsServices = new AdWordsServices();
AdWordsSession session = AdwordsUtils.getSession();

TrafficEstimatorServiceInterface trafficEstimatorService =
    adWordsServices.get(session, TrafficEstimatorServiceInterface.class);