If the pre-authorised amount and the final transaction amount differ your bank may treat this as two transactions and the bank may hold the amount of the pre-authorised amount until the transaction clears. It can take 5-10 working days (depending on bank) for the lien to be removed but please be assured you will only pay the final transaction amount. Where the pre-authorised and the final transaction amount match there is no issue.

You will only be charged for the amount the car actually takes. For example if you pre select €50 and your car only takes €40 then your account will only be debited for the €40.