Originally Posted by
TallestHotelInJapan
the newly added "prepaid fuel" (was neither booked nor mentioned on the original booking").
Which you agreed to, presumingly. If you signed a contract when you picked up, there was a "FSO" or Fuel Service Option, which you have to check indicating which Fuel option you want (you can decline all). OR the agent asks you about fuel when you pick up.
Either way, as
Often1 said - dont leave the counter unless you review your estimated charges when you pick up.