Thanks for the data points.
It seems Avis, like National, automatically applies the corporate code on any bookings when logged in and you don't have a choice to not use it unless you "remove" the code? (for good?)

The corporate rate I have is cheaper 95% time with the 5% when I need a one way rental with lots of mileage. I learned the hard way by getting charged like $0.25/mile overage on a rental earlier this year.