Not worth wasting 10 minutes on. Send in a webform complaint, wait 5 business days. If you don't see the credit or at least hear that it's coming, dispute the charge. Make sure you reference the two fill ups by transaction # (presuming that you used the same card to rent the vehicle and purchase the gas).

True that you may lose, but at least you will have tried.

For the future, just like business travel, throw all your receipts in an envelope and hang onto the envelope for some reasonable period of time after return. Not just gas, but to deal with anything untoward that comes your way).