Rentalcars.com and Flex To Go Charged 500 Euro After Rental Return

I made an SUV reservation via Rentalscar.com with Flex To Go at Düsseldorf Airport for the period 11–14 June 2026. After I had already completed my rental and returned the vehicle, I was unexpectedly charged an additional 500 Euro on 17 June 2026. I did not intentionally drive abroad. My route was simply from Aachen to Monschau, and the road apparently passed briefly through Belgium without any clear warning signs or information that I was leaving Germany, so I was not aware that I had crossed the border. I contacted customer service regarding this issue, but so far I have not received any response. I request that this 500 Euro extra charge be fully refunded and that I be informed in writing about how this amount was calculated and why it was charged under these circumstances.