I can't understand why this is is being dragged out. If all the details are as you say they are, then it really shouldn't matter what time your stated return time vs. your actual return time was, as log as both times are within 24 hrs of pickup. There have been a few times where I have booked partial days with Avis and ended up being a few hours late, but still within 24 hrs... I initially would call Avis and have them push back the agreed return time, but the phone agents always told me not to worry about it, so eventually I just stopped calling and brought back the car when I could, taking care to be within the 24 hr timeframe. Evn though I was past my return time, I was always charged a flat 1-day rate.

Have you called and asked to be transferred to Customer Service? They are pretty good about seeing things your way, especially if you are a frequent renter. This issue just seems so cut-and-dry I can't believe they are fighting it.