I hire Avis mainly in two locations; Oxford, UK and Munich, Germany. Never had any problems at Oxford, but have issues in Munich about 90% of the time.
1. Free second driver is often/usually charged for
2. Fuel charged for even though returned full
3. Voucher value doesn't match rental price.
so I now always make a point of;
a. Waiting to check the car in
b. Going to the counter to query/fix the charges (usually resolved then, sometimes not)
c. Email Avis (to fix charges) or BA (to fix Voucher-value problems).
It's a pain in the butt, but now I'm resigned to it. Munich had, for a time, an *awful* station manager(?) who would even refuse to look at the final invoice after checking the car back in.
The trouble is, are any other companies any better? And the "flight & car" pricing makes the trouble almost worthwhile.