Returning the car caught me out too. We ended up in the normal car park rather than the rental car park.
If this happens, don't worry, just note the space your in and give them 2 euros to cover the parking fee.
Two more things to note: Expect them to 'swipe' your payment card manually, this they will tear up if you return the car un-damaged. Second, you are responsible for tyre damage so no light off roading
Congrats with the DIY, short haul there's often a bit to be saved, long haul DIY's like Orlando saves you a fortune. Thanks to sites like this we can all be our own travel agent.