Ok....thanks for the tips.

I finally got through to the Avis ITH desk (after several days of trying but the phone kept giving me the national number).

They do have a metal drop-box there, and they said to also fill out the white sleeve that the rental info comes in so they can figure out what to charge me. (Put the gas level, mileage, etc.) Why do I have to write that stuff? Are they just going to believe me when I write that I have left the car with a full tank?