If you want to fight it, I’d email them and tell them you’ve 'rented cars many, many times over the past few years as I am a very frequent traveler" then go on to say that you’ve never seen this policy in the past. Next, go to the websites of Enterprise, Avis, Budget and anyone else you can think of and Copy/Paste their policy for returning the car. Apologize and say something like “I’m sorry, I just assumed it was the same with you, like I said I’ve rented many times [even if you haven’t] and this is how I’ve always done it. You said yourself it was ‘12 miles less then full’ so it’s not like I returned it a quarter or a half down and as for the receipt, I was not told to bring it back when I picked up the car and I was not asked for it when I dropped the car off.”
I would then go on to say…“Is there anyway you could refund the $30 I was charged for this, if you can, I’d be happy to continue using you, the prices were great and the car was nice. Please let me know if there’s anything you can do to help me out here, I know you have to draw the line somewhere, but $30 is a lot to charge for 2 miles worth of gas.”
I might even throw in “I feel like this may have been on purpose, someone at the desk should have asked for my receipt. How many people filled up at the gas station across the street and don’t turn in their receipt and get charged $30? On top of that if I had handed the clerk my receipt he could have mentioned your policy to me and I would have been more then happy to drive to the gas station across the street and put literally $1.00’s worth of gas in the car to save the $30.00 charge” But you might save this for after they decline your request, it’s going to make them defensive.
But yeah, send them an email, tell them you travel a lot, they might reverse it.
ETA, none of this’ll work if Enterprise/Avis etc all of the same policy (which they may, I have no idea).