Never had a problem with Firefly or Centauro. (Located in airport)
Stay away from Goldcar!!! (My experience)
I always book through Doyouspain.com. Cheaper than direct with company.
They offer full full option and full empty part refund, and refund in eighths of a tank.
Rentals appear to be cheaper on full empty/ refund option too, fill up before you take car back, and take photos of mileage and fuel gauge.
I have an annual insurance policy with icarinsurance.com so just let the rental company put a block on my credit card.
the problem with July rentals, like anything else is supply on demand.
I hired in November and only cost 26 euros for ten days!!!
hope that is of some help.
it would be good to find the ideal! As an upfront honest hire company could get a lot of business from this forum alone.