Budget responded within 4 hours to our email and refunded the fuel charge (at least we think so, will need to check the statement credit in a week).
They commented,
"Please note, to avoid this charge on future rentals, please ensure that you provide your fuel receipt at return."
Great. Now one more thing in addition to getting a fuel receipt, checking the final price, taking a photo of the gas and odometer, and filming the condition of the car. We rent cars about 8 times a year and over 10 years a rep has never asked us to provide the fuel receipt even though they check the fuel gauge.
@SandyPGravel Taking photos upon return is just as (if not more important) than leaving the lot because if there is any damage caused by a subsequent renter you have no proof that it was not you who caused the damage.