Out of interest, why do so many people feel they're getting ripped off with car hire? To me it's a necessary expense of owning at CDA, like flights, airport parking etc.
Is prepaying for fuel such a rip-off? We know we'll be charged full price per litre plus IVA (legally required because it's a service they are providing). If you don't want to pay it, there are plenty companies will rent you a car where you take it back full. Then you have to find a fuel station somewhere near the airport on the way back to save maybe 20euros. Trouble with that is the car hire rate for this option seems to be more in the first place so I doubt you really gain anything. For me it's easier just to pay up front and forget it.
Similarly with insurance "extras". If you take out a decent annual cover it will include CDW, PAI etc so you just decline them at the desk (unless they are already included, again you know that when you make the booking)