Don't know about the hidden service fees you mention (can you explain them a bit more maybe?) but we have used SIXT a couple of times from Alicante airport, but it was a couple of years ago - you can normally get a price direct from their site without going through a broker.

I don't think they were the cheapest, but we had none of the dodgy stuff when we returned the car, like you get with some of the cheepos like Goldcar, where they charge 300 euros to your credit card for a scratch you have alledgedly made, that deep down you suspect was there all the time, but didn't notice on pick up from a very dark garage after travelling 10 hours.

We do always take the daily top up insurance not sure if that helps (is that what you maybe mean by the hidden fee, it's something like 6 or 7 euros a day?)

Pretty much all of the companies we checked charge for a full tank of fuel plus a €30+service charge and then refund any fuel left.
I'll have a look at Sixt as I don't think ilI checked there.
I always take out a separate excess policy (rather than the hire companies one) to be on the safe side.

Also have a look at http://www.carjet.com as you can choose how you want to return car with fuel from the list on the left.

I don't drive so am not recommending the above just thought it may be worthwhile having a look.

Thanks. We're on the verge of just getting transfers and using the buses/trams to explore of we don't find something acceptable The upside of that is I can have a drink