I believe Carjet is just a broker and the rental agencies are where you collect your vehicle on arrival. Which one did you use?
I've found Carjet convenient and good value in the past, though about a month ago, when collection was from the Centauro desk rather than Europa Rentacar, there were long queues (discussed at length below in another thread - Hire car problems - Holiday autos / Centauro).
I've just checked my most recent booking confirmation with Carjet for this August and it clearly states that while the customer needs to pay for the first tank of fuel, they return their car with an empty tank, so if your booking was the same, you shouldn't have had to refill the tank.
It's confusing - I'm pretty certain that in previous years the rule was to fill the tank before returning the vehicle.
They probably supplement their revenue this way, relying on customers losing the paperwork and not remembering what to do. I'll be sure to take a copy of the booking confirmation next time!