The admin fee could be for something else
e.g. Hertz was contacted regarding something like speeding or not paying the toll for Dartford River Crossing and this is the fee for Hertz to pass details on
Have you tried making a telephone call to Customer relations at Hertz UK which may well be able to answer the question immediately
If it cannot answer the question, it can open a ticket for the issue and should be answered within 7-10 business days
I wouldn't use email for such a query
Dave