My last two Hertz rentals at SFO involved the same scam. Both times I pre-paid at Hertz.co.uk and both times Hertz at SFO, on the car's return, charged me significantly more, claiming that the voucher did not cover the entire rental cost.

On the first occasion, I thought it was a genuine mistake and spent 30 minutes at SFO before getting it refunded (it's just as well I like to arrive early for my flights). On the second occasion, both employees said they could not refund it without a manager, and there was none on duty - by which time I was getting jittery about my flight.

Kudos to Hertz - the manager did subsequently call me but claimed that there was nothing he could do, the money was owed as the UK voucher did not cover the full cost of the rental. I then called Hertz UK who immediately refunded it, saying that this seems to be a common problem with rentals there and they have had over a thousand similar issues where they have refunded money.

So - be careful if you are renting at SFO with a UK voucher.