I usually get the 500 miles but have not yet had the extra bonus. My claim is currently under investigation ...
I book online in the UK at fixed rates and have NEVER EVER had a bill for the correct amount. Usually 25% - 100% over the correct figure and always from 'automated' European city locations so this is not the excuse.
As I am a Founder Shareholder, Avis are always very helpful and never quibble about putting things right over the phone, but it is irritating to have to call each time.