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.

Carjet are a basically a good company to go through although there are odd quirks.  My son recently had a car from them and the amount you pay when booking covers all insurance claims, there is no excess to pay when you pick up the car from Solmar.  Also petrol has to be paid for at the time of renting but the charges are not the excessive amounts you would have to pay if you returned the car with an empty fuel tank to any of the other companies.  Also a guest at our property ordered a car from them and when he went to pick it up he had been upgraded to a better model and got a brand new car into the bargain.

The only problems with Carjet are, as my son found out, that they only take credit card bookings and as he refuses to hold a credit card, guess who had to produce theirs.  I believe that you can pay cash at the time of pick up but they don't take debit cards. And although he received a quote which was supposed to be valid for upto 3 days, when I came to pay the quote had risen ( about £1-50), even though it was only the following day.
When my son queried it with them they told him that it is normal for quotes to rise and would not give a refund.

There are 2 car hire places in Tortosa .  VH10 on the main roundabout as you approach Tortosa from C12 coming from Aldea

or Ebro Rent which is near bus station Tortosa can't rember the name of the road.

Possible they will accept cash.

Yes the spanish car hire  is more expensive it was just a suggestion as an alternative

If it's helpful:
I have never had problems with carjet / solmar at Reus Airport, and am pretty sure I have always paid (on arrival) both the hire charge and the deposit with a direct debit card - but obviously one with a visa logo ...