cbrown5294, this unfortunatelly sounds very familiar to me.
I have over 100 rentals with SIXT a year and nearly every time when returning a car and due to flight departure in hurry and no time to wait for check-in report they charge me fuel, though I always fill the cars up.

Totti, I would love to say, that I can second your experience, but itīs different in my cases.

Still, SIXT always accepted my complaint when sent via mail including the petrol station receipt and retransfered the charged money for the fuel-fee.

So just send them a mail and they will react pomptly.