They are working with different rental companies which are not all honest and reliable.
First time I booked with them all went smoothly.
Second time I booked it was Firefly. They are scammers. They are systematically fleecing the customers by adding extra costs and insurance even if a full insurance package is booked. So the final amount is way higher than the amount on indicated on the voucher. I have informed carflexi about this but they don't feel resposible.