If you have taken out a full empty car hire contract, they will charge you a refilling charge plus the fuel used at a slightly higher cost, take it back full and you only pay the refilling/administration charge of 30 odd euros, also make sure you take a credit card with you or they will make you take out their own insurance, if you read all the two page contract from doyouSpain it’s all in there.
take photos and inform them of any damage not on sheet, especially under side cills and front bumper
im not a lover of Goldcar.