No Feedback on additional charges levied

Rented a car from SIXT car rental in the UK from 30 July 2019 to 7 August 2019. Received an invoice with an extra charge levied for "minimum excess" of GBP142.47. Have queried in writing why this charge was levied as there were no accident claims or damages on the vehicle when it was returned. No responses or reply from SIXT rent a car despite using contact e-mail details provided. Tried calling on number provided but call cuts off. Very poor service from an "international company".