Mike,
It has been my experience that even though I see free transactions for channel installs, I do not get the push notifications for them.
However:
I was testing the "issue service credit" endpoint by attempting to issue service credits of $0.01 to free transaction ids. These failed as I expected (since the transaction was originally $0), but I still received push notifications for these attempts. This actually isn't ideal in the live channel, since I haven't figured out a way to determine after the fact if the credit was successful. It does give a way to test the push notifications though....