Thank you Amy, that form worked and someone from Google got back to me and clarified things.
It seems a charge failed sometime in the recent past and so it was never paid. The charge however wasn't showing in the Balance column so it appeared to me as if everything was OK. When billing was disabled, that outstanding charge popped up.
This is not the first time I've been hit by this, my employer had something similar happen when billing was disabled on an app. A charge had failed almost a year prior but was not showing in the balance of the app. Only when billing was disabled did it appear.
I consider this a bug. :(
Also, as for the whole "disabled the app but not the billing" thing, well that's a bug too ;).
Thanks again,
Marc