Avis Surcharges

I recently rented a car from AVIS in Brindisi Airport, Italy – I was charged a 346 euro of damage (I wont get into that drama here!) ... This included a damage processing charge of 65 euro. I had car rental excess insurance separately - so claimed this amount back recently.

However, avis also charge an airport surcharge and road user fee, both calculated as a % figure. While I would have thought these charges are just on the basic rental amount for the car, they have told me they calculate the % of total of all three thngs (the rental amount + damage + damage processing fee). This inflates the amount alot as you can imagine.

The car rental excess has not refunded this amount (as they cap damage processing fee and don’t refund other fees).

I was wondering if anyone had a similar experience? is this the norm? and how Avis can justify a location surcharge and road user fee as a % of both damage and damage fee rather than just on the basic rental amount.