Hi warner, try carjet.es if you are in UK just put in carjet on pc and give them a try, I have found them very good, cheap and reliable agents, and if you fly into Reus they normally give a car from Solmar cheaper than if you go to Solmar direct.
I know that you can prepay or pay at the airport but you might have to take an extra excess against loss or damage without a credit card, whether you can pay cash or not I do not know. Find out by getting in touch with them, you'll get a quick answer back.
Hope that helps,
                            old nelson.