We tend to use Goldcar via "DoYouSpain" who do not charge for cancellations or modifications to bookings.
For long holidays we tend to hire for a week at the beginning and a week at the end using an airport based hire company. Its a good excuse for a day in Palma, dropping off and picking up the cars. If we have one longer it tends to go unused for days. It s cheaper to get a bus or taxi if you need to shop etc.
Goldcar and Record dont always get good reviews, but I have had no problems so far.
As Brisbane mentioed, the daily rate is very low out of season if you can resist their additional insurance. But note... the yearly Insurance4carhire has a 31 day max per rental.
Its worth shopping around online to check prices via various brokers rather than going direct. Good Luck and let us know how you get on