I think the true test of a company is what they do when something goes wrong. Well, in my experience, regrettably, I have to report the answer is - not alot.
What would you have liked them to do and did you ask them to do this and did they refuse?
It strikes me that having:
1) Acknowledged that it was their mistake,
2) Refunded your money,
3) Agreed to pay the difference between what you had originally paid them and what you had to pay in resort,
that there's not a lot more they could have done in the situation. Agreeing to recompense you for your phonecalls perhaps? Did you ask them if they would do this? And what was their response? Were you looking for additional compensation?
Mistakes can happen and agree with you that the issue is what they do next to remedy the mistake that is important and I think that, personally, assuming that you were able to arrange a car hire with another supplier I would have regarded this as an OK response especially since they seem to have agreed without question that it was their mistake, certainly not one that would mean I would never book with them again. In my experience, when these things happen it can just as easily be the car hire firm at fault as was the case for me when it turned out that despite accepting a booking for a people carrier via TA they didn't actually have one in their fleet! With the result that 6 of us spent the week rattling around in a 14 seater minibus - at least there was plenty of room for the luggage!
SM