I ran into something like this on my last rental. I picked up my car earlier than I expected (everything went smooth) So, after I drove off and looked at my paperwork it was more than my quote. When I asked I was told that since I picked up my car early the computer charges in 24 hour increments, so I was charged an extra day! Were Talking picking up my car 30 minutes early and then wanting to return it two hours later than the pick up time. They told me they don't have a grace period, because people were taking advantage of this! Well, I had other issues and had too return the car because it did not run well. They will work with you at the little booth. They did take some off, so I guess it just depends on who you get at the booth and how much they can change. What kills you in Vegas is my day rate was $11 a day (which is not bad) Then you add the taxes and it's more than the car? The person I spoke too at the Car Rental Facility was nice, But before I returned the car I called a 1-800 number and actually he was rude! He did not like my question as if you are going to charge me for 10 mintutes why don't you credit me when I return a vehicle early? He Actually hung up on me?
So, this does not answer your question? But mybe it will help? I was so frustrated with Dollar that I went ahead and returned it to fall in the 24 hour time line and just went too the airport early and walked thru shops.
Dena